Biostimulator having fixation element

ABSTRACT

A biostimulator, such as a leadless cardiac pacemaker, including a fixation element to engage tissue and one or more backstop elements to resist back-out from the tissue, is described. The fixation element can be mounted on a housing of the biostimulator such that a helix of the fixation element extends distally to a leading point. The leading point can be located on a distal face of the helix at a position that is proximal from a center of the distal face. The backstop elements can include non-metallic filaments, such as sutures, or can include a pinch point of the biostimulator. The backstop features can grip the tissue to prevent unscrewing of the fixation element. Other embodiments are also described and claimed.

This application claims the benefit of priority of U.S. ProvisionalPatent Application No. 62/582,125, filed on Nov. 6, 2017, U.S.Provisional Patent Application No. 62/637,257, filed on Mar. 1, 2018,U.S. Provisional Patent Application No. 62/646,247, filed on Mar. 21,2018, U.S. Provisional Patent Application No. 62/700,112, filed on Jul.18, 2018, and U.S. Provisional Patent Application No. 62/750,034, filedon Oct. 24, 2018, each of which is incorporated herein by reference inits entirety to provide continuity of disclosure.

BACKGROUND Field

The present disclosure relates to biostimulators. More specifically, thepresent disclosure relates to leadless biostimulators having tissueanchors.

Background Information

Cardiac pacing by an artificial pacemaker provides an electricalstimulation of the heart when its own natural pacemaker and/orconduction system fails to provide synchronized atrial and ventricularcontractions at rates and intervals sufficient for a patient's health.Such antibradycardial pacing provides relief from symptoms and even lifesupport for hundreds of thousands of patients. Cardiac pacing may alsoprovide electrical overdrive stimulation to suppress or converttachyarrhythmias, again supplying relief from symptoms and preventing orterminating arrhythmias that could lead to sudden cardiac death.

Cardiac pacing by currently available or conventional pacemakers isusually performed by a pulse generator implanted subcutaneously orsub-muscularly in or near a patient's pectoral region. Pulse generatorparameters are usually interrogated and modified by a programming deviceoutside the body, via a loosely-coupled transformer with one inductancewithin the body and another outside, or via electromagnetic radiationwith one antenna within the body and another outside. The generatorusually connects to the proximal end of one or more implanted leads, thedistal end of which contains one or more electrodes for positioningadjacent to the inside or outside wall of a cardiac chamber. The leadshave an insulated electrical conductor or conductors for connecting thepulse generator to electrodes in the heart. Such electrode leadstypically have lengths of 50 to 70 centimeters.

Pacemaker leads can be fixed to an intracardial implant site by anengaging mechanism such as an anchor. For example, the anchor can screwinto the myocardium.

SUMMARY

Although more than one hundred thousand conventional cardiac pacingsystems are implanted annually, various well-known difficulties exist,of which a few will be cited. For example, a pulse generator, whenlocated subcutaneously, presents a bulge in the skin that patients canfind unsightly, unpleasant, or irritating, and which patients cansubconsciously or obsessively manipulate or “twiddle.” Even withoutpersistent manipulation, subcutaneous pulse generators can exhibiterosion, extrusion, infection, disconnection, insulation damage, orconductor breakage at the wire leads. Although sub-muscular or abdominalplacement can address some concerns, such placement involves a moredifficult surgical procedure for implantation and adjustment, which canprolong patient recovery.

A conventional pulse generator, whether pectoral or abdominal, has aninterface for connection to and disconnection from the electrode leadsthat carry signals to and from the heart. Usually at least one maleconnector molding has at least one terminal pin at the proximal end ofthe electrode lead. The male connector mates with a corresponding femaleconnector molding and terminal block within the connector molding at thepulse generator. Usually a setscrew is threaded in at least one terminalblock per electrode lead to secure the connection electrically andmechanically. One or more O-rings usually are also supplied to helpmaintain electrical isolation between the connector moldings. A setscrewcap or slotted cover can be included to provide electrical insulation ofthe setscrew. This briefly described complex connection betweenconnectors and leads provides multiple opportunities for malfunction.

Other problematic aspects of conventional pacing systems relate to theseparately implanted pulse generator and pacing leads. By way of anotherexample, the pacing leads, in particular, can become a site of infectionand morbidity. Many of the issues associated with conventionalbiostimulators, such as cardiac pacemakers, are resolved by thedevelopment of a self-contained and self-sustainable biostimulator, orso-called leadless biostimulator. The leadless biostimulator can beattached to tissue within a dynamic environment, e.g., within a chamberof a beating heart.

According to a first aspect of the invention a biostimulator comprises ahousing having a longitudinal axis and containing an electronicscompartment; and a fixation element mounted on the housing, wherein thefixation element includes a helix extending along a helical axis aboutthe longitudinal axis to a distal edge, wherein the distal edge extendsaround the helical axis and defines one or more helix faces on thehelical axis, wherein a transverse plane orthogonal to the longitudinalaxis intersects a center of the one or more helix faces, and wherein thehelix includes a leading point on the helix face proximal to thetransverse plane.

Advantageously, the helix can include an ellipsoidal outer surfaceextending around the helical axis. Moreover, the distal edge can be atan intersection between the ellipsoidal outer surface and the one ormore helix faces. Further, the leading point can be on the distal edge.

Furthermore, it can be intended that a longitudinal plane can intersectand be orthogonal to the transverse plane at the center of the one ormore helix faces. Further, the leading point can be at a six o'clockposition on the longitudinal plane.

In another advantageous embodiment of the invention the six o'clockposition can be on the distal edge.

Advantageously, the one or more helix faces can include a plurality ofbevel faces converging at the leading point. Moreover, the plurality ofbevel faces can intersect along a leading edge extending from theleading point along the one or more helix faces to a base on the distaledge. Further, the base can be on an opposite side of the transverseplane from the housing.

It can be further intended that the biostimulator can further comprise ahelix mount having a helix mount flange, wherein the helix mount flangecan include one or more marks defining an alignment range. Moreover, thehelix can extend distally from a distal end of the helix mount flange tothe leading point. Further, the leading point can be aligned with thealignment range.

In another advantageous embodiment of the invention the one or moremarks can be radiopaque such that the one or more marks are visibleunder fluoroscopy.

Furthermore, it is thereby possible that the alignment range can bebetween a leftward boundary and a rightward boundary of the one or moremarks. Moreover, the helix can extend to the leading point that isvertically aligned with the alignment range.

Advantageously, the biostimulator can further comprise a plurality ofbackstop elements to resist backward movement of the housing when thefixation element is engaged in tissue.

It can be further intended that the plurality of backstop elements caninclude a non-metallic filament extending through a bore in a sidewallof the helix mount along a filament axis to a filament tip.

Advantageously, the filament tip can have a filament face on thefilament axis. Moreover, the filament face can be at an angle to thefilament axis.

Furthermore, it can be intended that the plurality of backstop elementscan include a plurality of non-metallic filaments extending throughrespective bores in the sidewall. Further, each bore can be at adifferent longitudinal position relative to the longitudinal axis.

Advantageously, the non-metallic filament can include a natural fiber.

It can be further intended that the plurality of backstop elements caninclude a pinch point between the helix and the distal end of the helixmount flange.

Advantageously, the biostimulator can have a first removal torque whenthe fixation element is engaged in tissue and the tissue is not at thepinch point. Moreover, the biostimulator can have a second removaltorque when the fixation element is engaged in the tissue and the tissueis at the pinch point. Further, the second removal torque can be atleast 10% higher than the first removal torque.

Furthermore, it can be intended that the biostimulator can be a leadlesscardiac pacemaker.

Advantageously, the biostimulator can further comprise an active helicalelectrode mounted on the housing. Moreover, the active helical electrodecan include an electrode helix extending along an electrode helical axisabout the longitudinal axis radially inward of the helical axis.Further, the electrode helical axis and the helical axis can revolveabout the longitudinal axis in a same rotational direction.

According to a further aspect of the present invention a method is madeavailable, which comprises mounting a helix mount on a housing, whereinthe helix mount includes a helix mount flange, wherein the helix mountflange includes one or more marks defining an alignment range; andscrewing a fixation element onto the helix mount, wherein the fixationelement includes a helix extending along a helical axis, and wherein thehelix is screwed onto the helix mount flange until a leading point ofthe helix is aligned with the alignment range.

Moreover, it is possible according to the present invention that the oneor more marks can be radiopaque such that the one or more marks arevisible under fluoroscopy.

Advantageously, the alignment range can be between a leftward boundaryand a rightward boundary of the one or more marks. Moreover, the helixcan extend over about 1.4 to about 1.6 turns of the helical axis to theleading point when the leading point is vertically aligned with thealignment range.

Furthermore, it is thereby possible that the method can further compriseinserting a non-metallic filament through a bore in a sidewall of thehelix mount.

Advantageously, the helix mount can include a cap mounted distally onthe helix mount, and the bore can be in the cap.

Moreover, it is possible according to the present invention that thehelix can extend along the helical axis about a longitudinal axis of thehousing to a distal edge. Moreover, the distal edge can extend aroundthe helical axis and defines one or more helix faces on the helicalaxis. Further, a transverse plane orthogonal to the longitudinal axiscan intersect a center of the one or more helix faces. Beyond that, theleading point can be proximal to the transverse plane.

Furthermore, it can be intended that the helix can include anellipsoidal outer surface extending around the helical axis. Further,the distal edge can be at an intersection between the ellipsoidal outersurface and the helix face. Beyond that, the leading point can be on thedistal edge.

Beyond that the method according to the invention can further compriseadvancing a biostimulator to a target tissue, wherein the biostimulatorcan include a housing having a longitudinal axis and containing anelectronics compartment, and a fixation element can be mounted on ahelix mount flange of the housing, wherein the fixation element caninclude a helix extending from a distal end of the helix mount flange toa leading point, wherein the helix mount flange can include one or moreradiopaque marks defining an alignment range, and wherein the leadingpoint can be aligned with the alignment range, viewing the one or moreradiopaque marks under fluoroscopy and rotating the biostimulator untilthe fluoroscopically viewed one or more radiopaque marks complete apredetermined number of turns.

Advantageously, the predetermined number of turns can be about 1.5turns.

In the following, the invention is described in more detail, whereby allfeatures of the following description may refer to the previously listedexplanations.

A biostimulator for implantation within a heart of a patient can beprovided. Preferably, the biostimulator is a leadless biostimulator. Inan embodiment, the biostimulator can comprise an electronicscompartment. Additionally or alternatively the biostimulator can includea housing containing an energy source, e.g., a battery, anultracapacitor, or an energy harvester, and the housing can have alongitudinal axis. Additionally or alternatively the biostimulator caninclude a housing comprising electronic circuity and an energy source.The biostimulator can include a fixation element mounted on the housing.For example, the fixation element can be a helix, formed from a wire,that extends about the longitudinal axis to a leading point. The leadingpoint can penetrate tissue to anchor the biostimulator within the heart.Additionally or alternatively the leading point can be on a helix facethat intersects an outer surface of the wire along a distal edge.Additionally or alternatively the leading point can be on the wire faceat a location that controls a depth of penetration of the wire when thefixation element is screwed into target tissue. For example, the leadingpoint can be on the wire face between a center of the wire face and thehousing, e.g., proximally located at a six o'clock position on thedistal edge. When the leading point is at the six o'clock position, thefixation element may penetrate more shallowly than when the leadingpoint is at a twelve o'clock position diametrically opposite the sixo'clock position on the distal edge. It has been discovered that shallowpenetration can be useful when anchoring in tissue substrates, and moreparticularly, when anchoring in anatomical structures that are layered.For example, when several tissue substrates are immediately adjacent,such as in the case of the atrium covered by the pericardial sac,limiting penetration depth of the leading point can reduce a likelihoodof injury and can improve device performance. The leading point can bebetween a transverse plane and the housing to cause fixation element topenetrate less deeply into target tissue. More particularly, leadingpoint can be proximal to the transverse plane. By contrast, leadingpoint can be on an opposite side of transverse plane from housing, e.g.,distal to the transverse plane, to cause fixation element to penetratemore deeply into the target tissue. Transverse plane can be orthogonalto longitudinal axis (or helix axis) and may extend along a median lineof wire face. More particularly, transverse plane can intersect a centerof wire face. Accordingly, transverse plane can define a longitudinallylocated separator between a portion of wire face that is proximal most(the surface area that is between transverse plane and housing) and aportion of wire face that is distalmost (the surface area that is on anopposite side of transverse plane from housing). The fixation elementcan include a helix extending along a helical axis to a helix face.

The biostimulator can optionally include several backstop elements toresist backward movement of the housing when the fixation element isengaged in tissue. For example, the backstop elements can includenon-metallic filaments that extend outward from a helix mount that holdsthe fixation element. The filaments can be sutures, natural fibers,etc., which engage the tissue in an opposite direction from the fixationelement, and thus, increase a back-out torque required to dislodge thebiostimulator from the tissue.

The backstop elements can include a pinch point between the wire and thehelix mount. More particularly, when the fixation element is screwedinto the tissue by a predetermined amount, e.g., 1.5 turns, the tissuecan be clamped between the wire and the helix mount at the pinch point.The clamping and/or scarring caused by the pinch point can furthersecure the biostimulator within the dynamic operating environment.

The biostimulator can include one or more marks that facilitate properassembly of the fixation element. More particularly, the one or moremarks can have boundaries that define an alignment range, and placementof the leading point within the alignment range during assembly canensure that tissue is captured at the pinch point when the fixationelement is screwed into the tissue by the predetermined amount, e.g.,1.5 turns. In an aspect, the marks can be used during manufacturing byscrewing the fixation element onto the helix mount until the leadingpoint is vertically aligned between a leftward boundary and a rightwardboundary. The mark(s) can also enable accurate implantation in aclinical setting.

For example, one or more of the mark(s) may be radiopaque, and thus, canbe visible under fluoroscopy or using another imaging modality. Theradiopaque mark(s) can be viewed during a clinical implantation of thebiostimulator to assess a degree of rotation of the fixation elementinto the target tissue. For example, an operator can advance thebiostimulator to the target tissue, view the radiopaque mark(s) underfluoroscopy, and rotate the biostimulator until the fluoroscopicallyviewed mark(s) complete a predetermined number of turns. The number ofturns can be a nominally recommended number, e.g., 1.5 turns.Alternatively, the number of turns can be fewer than the nominallyrecommended number, e.g., 1 turn, when the tissue strength is known orexpected to be degraded such as when the tissue is friable, the tissueis in a diseased state, or the patient has had prior cardiac surgeriesor procedures. Under-rotation (as compared to a nominal recommendedrotation) may be beneficial in tissue that is not able to withstand theincrease in torque that occurs as a device is rotated toward and beyondthe pinch point. In an embodiment, the biostimulator can comprise anactive helical electrode nestled within a helical fixation element. Bothhelices may provide active fixation for the biostimulator. In certainembodiments, the wind direction can be the same for both helices.

The above summary does not include an exhaustive list of all aspects ofthe present invention. It is contemplated that the invention includesall devices, in particular the biostimulator, systems and methods, inparticular the method comprising mounting a helix mount on a housing,that can be practiced from all suitable combinations of the variousaspects summarized above, as well as those disclosed in the DetailedDescription below and particularly pointed out in the claims filed withthe application. Such combinations have particular advantages notspecifically recited in the above summary. Here, features, which aredescribed in conjunction with the biostimulator, apply, of course alsoin conjunction with the method according to the invention, and viceversa, so that, with respect to the disclosure, reference is or canmutually be made to the individual aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 illustrates a biostimulator in accordance with the presentdisclosure.

FIG. 2 is an isometric view of a biostimulator in accordance with thepresent disclosure.

FIG. 3 is a pictorial illustration of a distal face of a biostimulatorin accordance with the present disclosure.

FIGS. 4A-4E are schematic illustrations of interactions between abiostimulator and a target tissue in accordance with the presentdisclosure.

FIG. 5 is a distal view of a biostimulator in accordance with thepresent disclosure.

FIGS. 6A-6B are schematic illustrations of delivery of biostimulatorsinto a patient heart in which the biostimulators are to be implanted inaccordance with the present disclosure.

FIG. 7 is an exploded view of a header assembly of a biostimulator inaccordance with the present disclosure.

FIGS. 8A-8B are top and bottom views, respectively, of a cap of a headerassembly of a biostimulator in accordance with the present disclosure.

FIGS. 9A-9B are side elevation and cross-sectional side views,respectively, of a fixation element of a header assembly in accordancewith the present disclosure.

FIGS. 10A-10D are side elevation views of a fixation element inaccordance with the present disclosure.

FIG. 10E is a top view of a fixation element in accordance with thepresent disclosure.

FIG. 11A-11C are side elevation views of a fixation element inaccordance with the present disclosure.

FIGS. 12A-12C are side elevation views of a fixation element inaccordance with the present disclosure.

FIG. 13A-13C are side elevation views of a fixation element inaccordance with the present disclosure.

FIGS. 14A-14C are side elevation views of a fixation element inaccordance with the present disclosure.

FIG. 15 is an isometric view of a biostimulator in accordance with thepresent disclosure.

FIG. 16 is a distal view of the biostimulator in accordance with thepresent disclosure.

FIG. 17 is a distal view of a biostimulator having a fixation elementremoved in accordance with the present disclosure.

FIG. 18 is a side elevation view of a biostimulator in accordance withthe present disclosure.

FIG. 19 is a side elevation view of a biostimulator in accordance withthe present disclosure.

FIG. 20 is an isometric view of a biostimulator in accordance with thisdisclosure.

FIGS. 21A and B are lateral side views of a distal portion of abiostimulator in accordance with the present disclosure.

FIG. 22 is a distal end view of a biostimulator in accordance with thepresent disclosure.

FIG. 23 is an isometric view of a biostimulator in accordance with thepresent disclosure.

FIGS. 24A and B are lateral side views of a distal portion of theexample biostimulator in accordance with the present disclosure.

FIG. 25 is a distal end view of a biostimulator in accordance with thepresent disclosure.

FIG. 26 is a graphical view of a back-out torque of a biostimulator inaccordance with the present disclosure.

FIGS. 27A and B are pictorial views of biostimulator features that havean effect on penetration depth in accordance with the presentdisclosure.

FIG. 28 is a graphical view of implantation angle versus implantationrisks in accordance with the present disclosure.

FIG. 29 is a pictorial view of a fixation element having a leading pointin a twelve o'clock position penetrating tissue in a layered tissueenvironment, in accordance with the present disclosure.

FIG. 30 is a pictorial view of a fixation element having a leading pointin a six o'clock position penetrating tissue in a layered tissueenvironment, in accordance with the present disclosure.

FIG. 31 is an exploded cross-sectional view of a distal end of thebiostimulator, in accordance with the present disclosure.

FIGS. 32A and B are perspective views of a biostimulator having anactive helical electrode, in accordance with the present disclosure.

FIG. 33 is a pictorial view of a primary helix having a leading point atsix o'clock position, and a primary helix having a leading point at atwelve o'clock position, in accordance with the present disclosure.

DETAILED DESCRIPTION

Embodiments describe a biostimulator, e.g., a leadless cardiacpacemaker, having a fixation element that is mounted on a housing andincludes a helix extending to a leading point for piercing tissue. Moreparticularly, the wire can extend along a helical axis to a distal edgedefining a helix face. The leading point can be on the wire face betweena center of the wire face and the housing. The biostimulator may be usedto pace cardiac tissue as described below. The biostimulator may be usedin other applications, such as deep brain stimulation, and thus,reference to the biostimulator as being a cardiac pacemaker is notlimiting.

In various embodiments, description is made with reference to thefigures. However, certain embodiments may be practiced without one ormore of these specific details, or in combination with other knownmethods and configurations. In the following description, numerousspecific details are set forth, such as specific configurations,dimensions, and processes, in order to provide a thorough understandingof the embodiments. In other instances, well-known processes andmanufacturing techniques have not been described in particular detail inorder to not unnecessarily obscure the description. Reference throughoutthis specification to “one embodiment,” “an embodiment,” or the like,means that a particular feature, structure, configuration, orcharacteristic described is included in at least one embodiment. Thus,the appearance of the phrase “one embodiment,” “an embodiment,” or thelike, in various places throughout this specification are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, configurations, or characteristics maybe combined in any suitable manner in one or more embodiments.

The use of relative terms throughout the description may denote arelative position or direction. For example, “distal” may indicate afirst direction along a longitudinal axis of a biostimulator housing.Similarly, “proximal” may indicate a second direction opposite to thefirst direction. Such terms are provided to establish relative frames ofreference, however, and are not intended to limit the use or orientationof a biostimulator to a specific configuration described in the variousembodiments below.

In an aspect, a biostimulator is provided. The biostimulator includes afixation element to engage tissue for anchoring the biostimulator withina patient anatomy. The fixation element can include a helix extendingalong a helical axis to a helix face. A leading point of the wire can beon the wire face between a center of the wire face and a housing ofbiostimulator. Accordingly, the leading point can be within a proximalportion of the wire face. The proximally located leading point maypierce tissue more shallowly than a comparative leading point locateddistal to the center of the wire face. As a result, the fixation elementmay be less likely to extend fully through a target tissue.

In an aspect, a biostimulator optionally includes backstop elements toresist backward movement of the fixation element and/or housing of thebiostimulator when the fixation element is engaged in tissue. Thebackstop elements can include non-metallic filaments extending outwardfrom a mounting helix on the housing. The non-metallic filaments canresist back-out of the fixation element by grabbing tissue when thehelical wire of the fixation element unscrews from the target tissue.Furthermore, the backstop elements can include a pinch point between thewire of the fixation element and a distal end of the helix mount. Whenthe fixation element is screwed into the target tissue, the tissue canwedge between the fixation element and the helix mount at the pinchpoint, causing the biostimulator to clamp onto the tissue. The clampingforce can resist back-out of the fixation element.

In an embodiment, the biostimulator includes one or more marks on thehelix mount that define an alignment range for the leading point of thefixation element. More particularly, the fixation element can be screwedonto the helical helix mount until the leading point is aligned with thealignment range between the one or more marks. When so aligned, the wirecan extend over about 1.5 turns to the leading point. Set in such afashion, the fixation element provides anchoring in the target tissue,and the tissue can be clamped by the pinch point to resist back-out.

Various embodiments of a system including one or more leadlessbiostimulators, e.g., leadless cardiac pacemakers, are described. Forexample, the system can be a cardiac pacing system having a leadlesscardiac pacemaker. The leadless pacemaker can be substantially enclosedin a hermetic housing suitable for placement on or attachment to theinside or outside of a cardiac chamber. The pacemaker can have two ormore electrodes located within, on, or near the housing, for deliveringpacing pulses to muscle of the cardiac chamber and optionally forsensing electrical activity from the muscle, and for bidirectionalcommunication with at least one other device within or outside the body.The housing can contain a primary energy source, e.g., a battery, toprovide power for pacing, sensing, and communication (for example,bidirectional communication). The housing can optionally containelectric circuits. For example, the housed electric circuits can be forsensing cardiac activity from the electrodes. The housing can containcircuits for receiving information from at least one other device viathe electrodes and contains circuits for generating pacing pulses fordelivery via the electrodes. For example, a pulse generator can behermetically contained within the housing of the leadless pacemaker andelectrically connected to at least first and second electrodes of theleadless pacemaker. The pulse generator can be configured to generateand deliver electrical pulses via at least the first and secondelectrodes to cause cardiac contractions. The housing can optionallycontain circuits for transmitting information to at least one otherdevice via the electrodes and can optionally contain circuits formonitoring device health. The housing contains circuits for controllingthese operations in a predetermined manner.

In some embodiments, a cardiac pacemaker can be adapted for implantationinto tissue in the human body. In a particular embodiment, a leadlesscardiac pacemaker can be adapted or configured for implantation in atleast one cardiac chamber of a patient, e.g., adjacent to heart tissueon the inside or outside wall of the cardiac chamber. The leadlesscardiac pacemaker can be configured for leadless cardiac pacing, e.g.,the leadless pacemaker can use two or more electrodes located on orwithin the housing of the pacemaker to pace the cardiac chamber uponreceiving a triggering signal from at least one other device within thebody. Self-contained or leadless pacemakers or other biostimulators canbe fixed to an intracardial implant site by an actively engagingmechanism such as a screw or helical member that screws into themyocardium.

FIG. 1 shows a biostimulator 100. The biostimulator 100 can be aleadless biostimulator, e.g., a leadless cardiac pacemaker, for example.The biostimulators can include a hermetic housing 102 with electrodes104 and 106 disposed thereon. The electrodes can be integral to thehousing 102, or connected to the housing at a maximum distance of 2centimeters from the housing. Housing 102 can optionally contain anenergy source (not shown) to provide power to the pacing electrodes 104,106. The energy source can be a battery, such as a lithium carbonmonofluoride (CFx) cell, or a hybrid battery, such as a combined CFx andsilver vanadium oxide (SVO/CFx) mixed-chemistry cell. Similarly, theenergy source can be an ultracapacitor. In an embodiment, the energysource can be an energy harvesting device, such as a piezoelectricdevice that converts mechanical strain into electrical current orvoltage.

In certain embodiments, the energy source can be located outside of thehousing. For example, the energy needed to power the electrical circuitscould come from an ultrasound transducer and receiver, which receiveultrasound energy from an ultrasound transmitter located outside of thehousing.

As shown, electrode 106 can be disposed on or integrated within afixation element 105, and the electrode 104 can be disposed on thehousing 102. The fixation element 105 can be mounted on housing 102, asdescribed below. Fixation element 105 can be a fixation helix or otherflexible or rigid structure suitable for attaching the housing 102 totissue, such as heart tissue. Fixation element 105 may alternately bereferred to as a primary fixation element, a primary fixation helix, afixation helix, a primary helix, or similar. Such terminology may serveto distinguish from other, secondary, fixation elements that resistdislodgement of primary fixation element 105 after it has attached thehousing 102 to tissue. In other embodiments, the electrode 106 may beindependent from the fixation element in various forms and sizes. Thehousing can also include an electronics compartment 110 within thehousing that contains the electronic components necessary for operationof the biostimulator. By co-locating the stimulation electrode with thepacing generator, and by reducing the pulse generator size to fit withinthe heart, the biostimulator can be leadless. The hermetic housing canbe adapted to be implanted on or in a human heart, and can becylindrically shaped, rectangular, spherical, or any other appropriateshapes, for example.

The housing can include a conductive, biocompatible, inert, andanodically safe material such as titanium, 316L stainless steel, orother similar materials. The housing can further include an insulator108 disposed on the conductive material to separate electrodes 104 and106. The insulator can be an insulative coating on a portion of thehousing between the electrodes, and can include materials such assilicone, polyurethane, parylene, or another biocompatible electricalinsulator commonly used for implantable medical devices. In theembodiment of FIG. 1, a single insulator 108 is disposed along theportion of the housing between electrodes 104 and 106. In someembodiments, the housing itself can include an insulator instead of aconductor, such as an alumina ceramic or other similar materials, andthe electrodes can be disposed upon the housing.

As shown in FIG. 1, the biostimulator can further include a headerassembly 112 to isolate electrode 104 from electrode 106. The headerassembly 112 can be made from tecothane or another biocompatibleplastic, and can contain a ceramic to metal feedthrough, a glass tometal feedthrough, or other appropriate feedthrough insulator.

The electrodes 104 and 106 can include pace/sense electrodes, or returnelectrodes. A low-polarization coating can be applied to the electrodes,such as platinum, platinum-iridium, iridium, iridium-oxide,titanium-nitride, carbon, or other materials commonly used to reducepolarization effects, for example. In FIG. 1, electrode 106 can be apace/sense electrode and electrode 104 can be a return electrode. Theelectrode 104 can be a portion of the conductive housing 102 that doesnot include an insulator 108.

Several techniques and structures can be used for attaching the housing102 to the interior or exterior wall of the heart. A fixation element105, which can be a helical fixation element, can enable insertion ofthe device endocardially or epicardially through a guiding catheter. Atorqueable catheter can be used to rotate the housing and force thefixation element into heart tissue, thus affixing the fixation element(and also the electrode 106 in FIG. 1) into contact with stimulabletissue. Electrode 104 can serve as an indifferent electrode for sensingand pacing. The fixation element may be coated partially or in full forelectrical insulation, and a steroid-eluting matrix may be included onor near the device to minimize fibrotic reaction.

Biostimulator 100 may be used in a dynamic environment. For example,biostimulator 100 can be a leadless cardiac pacemaker placed within thedynamic environment of a beating heart, e.g., within an atrium or aventricle of the heart. When fixation element 105 is engaged with thecontracting and relaxing heart tissue, forces can be applied tobiostimulator 100 that may promote dislodgement, e.g., unscrewing, ofthe fixation element 105. Accordingly, in certain embodiments, alikelihood of dislodgement of biostimulator 100 may be reduced byincorporating anti-unscrewing features in biostimulator 100 that resistdislodgement.

Biostimulator 100 can include one or more backstop elements (FIG. 2),which can include various anti-unscrewing features on the biostimulator.The backstop elements are optional, e.g., biostimulator 100 can havefixation element 105 in accordance with this description without havingbackstop elements. The backstop elements can require that the torquenecessary to unscrew the biostimulator from tissue is greater than thetorque necessary to unscrew the biostimulator without such a feature.The backstop elements may also be referred to as secondary fixationelements because the backstop elements grab tissue to provide resistanceto back-out or rotation in an opposite direction to the rotationrequired to engage tissue with fixation element 105. Backstop elements,anti-unscrewing features, or secondary fixation elements, as alternatelyreferred to below, can include sutures, whiskers, or other means ofresisting, preventing, or stopping backward movement of housing 102 whenfixation element 105 is engaged in tissue. For example, a backstopelement can include a functional interaction between several componentsof biostimulator 100, e.g., between fixation element 105 and housing102, that pinches or clamps heart tissue to resist dislodgement ofbiostimulator 100 under dynamic conditions.

In some embodiments, the torque necessary to unscrew the biostimulatorfrom tissue is greater than the torque necessary to further screw,engage, or re-engage the fixation element 105 of the biostimulator 100into tissue. When a backstop element provides this function, the chancesof a biostimulator accidentally unscrewing or disengaging itself fromthe tissue is reduced. It should be noted that the torque necessary toinitially insert a biostimulator into tissue is greater due to thepuncturing or piercing of tissue and the formation of a helical cavity.Thus, in some embodiments, the anti-unscrewing features need onlyprovide that the torque necessary to unscrew the biostimulator fromtissue be greater than the torque necessary to unscrew the biostimulatorfrom tissue after the biostimulator has already been implanted in tissue(i.e., after the tissue has been pierced).

The effectiveness of biostimulator anti-unscrewing features may varydepending on the fixation location of the biostimulator within the heartand, more specifically, the shape of the tissue surrounding the fixationlocation. For example, each of the ventricular walls tends to define agenerally conical volume with the septum that tapers from theatrioventricular valve to the apex of the heart. During beating of theheart, contraction of the ventricular walls causes the distance betweenthe walls and the septum to narrow, particularly in the region closestto the apex. Such narrowing may result in the ventricular walls and/orthe septum laterally contacting a biostimulator fixed near the apex. Asa result, anti-unscrewing features extending laterally from thebiostimulator can provide sufficient anti-unscrewing performance tomaintain fixation of biostimulators within the apical region of theventricles.

In comparison to the generally conical/tapering shape of the ventricles,the atrial walls tend to define substantially rounder cavities such thatan atrial region in which a similar narrowing effect observed betweenthe ventricular walls and the septum is not generally present. Rather,the surface of the atrial walls remains substantially flat throughoutcontraction of the atria. As a result, biostimulators fixed within theatria are essentially flush mounted with the atrial wall such thatlaterally extending anti-unscrewing features may not engage the atrialwall and may not resist unscrewing of the biostimulator.

In light of the foregoing, biostimulators in accordance with thisdisclosure can include lateral or forward facing backstop elements thatprovide anti-unscrewing functionality even when the biostimulator issubstantially flush mounted with cardiac tissue. More specifically,biostimulators in accordance with this disclosure can include one ormore backstop elements disposed on a forward face of the biostimulatoradjacent a primary fixation element, such as a helical screw. Forexample, the backstop element(s) can include a non-metallic filament,such as a suture, extending in a lateral or forward direction from adistal region of biostimulator 100. The sutures are oriented in adirection at least partly opposite the primary fixation mechanism suchthat after fixation of the biostimulator by rotation in a firstdirection, counter rotation causes the sutures to engage tissue adjacentthe primary fixation mechanism, thereby resisting further counterrotation.

In certain embodiments, such as for the atrium, the biostimulator onlyincludes lateral or forward-facing back-up elements.

In certain embodiments, the biostimulator includes both side andforward-facing back-up elements, in order to configure the biostimulatorto be implanted in either an atrium or ventricle.

In certain implementations, the sutures are formed of a flexiblematerial such that sufficient counter rotational force applied to thebiostimulator may cause the sutures to bend and disengage from thetissue adjacent the primary fixation. As a result, the biostimulator maybe removed or repositioned from the fixation site with minimal damage totissue at the fixation site. Disengagement of one or more of the suturesmay also be controlled by positioning the sutures such that bending ofthe sutures during counter rotation is obstructed by the primaryfixation mechanism/helical screw. Other aspects of the presentdisclosure are directed to specific arrangements of sutures and methodsof coupling the sutures to the biostimulator housing.

It will be appreciated by one skilled in the art that pacemaker helicescan be terminated in a sharpened tip to facilitate initial puncturingand subsequent penetration into cardiac tissue. The sharpened tip can beformed at the distal extent of the helix with the sharpened tip forminga leading portion of the helix. Such tip arrangements may have goodtissue engagement and fixation characteristics, however, placement ofthe sharpened tip on the distalmost edge of the helix may increase thelikelihood that the helix will penetrate through the heart wall,particularly in thinner walled portions of the heart such as the atrium.In certain cases, penetration by the helix may result in unfavorableinteraction with the pericardium and complications such as cardiactamponade. In certain cases, for example, the helix may be screwedthrough the pericardium to the extent that it pierces the aorta.

Referring to FIG. 27A, biostimulator 100 can have several features thataffect anchoring of the device in the target tissue. For example, apitch 2701 defines a gap between helical turns within which tissue canbe captured during implantation. The gap can affect the anchoring, e.g.,by controlling a surface area to tissue ratio, and can also be avariable in determining how deep the anchor penetrates.

Referring to FIG. 27B, similarly, a projection 2703 of the leading pointof the anchor beyond a distal end of the biostimulator 100 can affectanchoring by determining how deep the helix will penetrate duringimplantation. In certain embodiments, the tip does not stand proud ofthe helix mount greater than tissue. In certain embodiment, the maximumprojection 2703 is 1.5 mm. An interference 2705 between the distal endof the biostimulator, e.g., the distal end of an electrode 2711, and thehelix proximal to the electrode can affect how much back pressure isplaced on the device by the target tissue, and thus, how well the deviceanchors. This is the distance between the distal end of the electrodeand the distal end of the helix mount.

Referring again to FIG. 27A, as described below, a placement of a tissuepinch point 2110 can affect how well tissue is gripped by the pinchpoint, and thus, how well the device is anchored to the tissue. Severalor all of these variables can be codependent, and can be affected by anumber of turns of the helical anchor beyond the pinch point. Moreparticularly, the number of turns can be a variable, which incombination with the other codependent variables, determines how wellthe device anchors to target tissue and a degree of injury to the targettissue. Accordingly, the number of turns correlates to a degree ofclinical risk, e.g., how likely the implanted device is to dislodge fromthe target tissue and how likely the implanted device is to cause injuryto the target tissue.

Referring again to FIG. 27B, in an embodiment, the primary helix 205 canhave a wire diameter 2750 from and including 0.003 inches to andincluding 0.03 inches. The primary helix 205 can have a pitch diameterfrom and including 0.06 inches to and including 0.3 inches. Pitch 2701can be from and including 0.01 inches to and including 0.05 inches. Theprojection 2703 of the leading point of the anchor beyond a distal endof the biostimulator 100 can be 0-1.5 mm. The interference 2705 betweenthe distal end of electrode 2711 and the helix proximal to the electrodecan be 0-1.5 mm.

In certain embodiments, the tissue pinch point 2110 tissue is wedged atthe pinch point 2110 during implant, as described herein by turning thehelix 1.5 turns. As described herein, the operator may, however, turnbiostimulator 100 to a more or lesser degree depending on the particularpatient. For example, when the patient has friable tissue, the operatormay choose to rotate biostimulator 200 by 1.25 turns instead of 1.5turns.

Referring to FIG. 28, a graph of device implantation angle versusclinical risk is shown. A range 2801 is shown which balances a risk ofdislodgement of a device after implantation against a risk ofperforating tissue, e.g., the pericardial sac. In an embodiment, therange 2801 is a range of turns in which a balance between the risks iseffectively minimized. For example, when an implantation angle isbetween 1-2 turns of the device during implantation, an acceptable levelof risk of dislodgement and injury may be achieved. Accordingly,rotating the fixation element 1.5 turns into the target tissue canprovide acceptable anchoring and an acceptably low risk of perforation.The implantation angle can depend on the multiple, codependent variablesdescribed above, as well as additional variables such as a placement ofa leading point of the fixation element, as described below.

In an embodiment, the tips of biostimulators in accordance with thisdisclosure may be oriented to facilitate engagement and retention of thebiostimulators within cardiac tissue while reducing the likelihood ofexcessive trauma to the heart. As described above, shallow penetrationof the fixation element 105 can be advantageous in certain anatomicalenvironments. Furthermore, the depth of penetration can depend on aclocking of a leading tip of the fixation element 105. The clockingconcept is referred to throughout the following description, and thus,an introduction to the concept is provided here. Referring to FIG. 33, apictorial view of a primary helix 205A having a leading point 1052 atsix o'clock position 3301, and a primary helix 205B having a leadingpoint 1052 at a twelve o'clock position 3303, are shown side by side forcomparison purposes. The positions of the leading point 1052 can beunderstood with reference to a clock 3305. The clock 3305 can have atwelve o'clock location at the top of the clock, which can correspond toa distalmost location in relation to the longitudinal axis 304.Similarly, the six o'clock location at the bottom of the clock cancorrespond to a proximalmost location in relation to the longitudinalaxis 304. Accordingly, the six o'clock position 3301 can belongitudinally proximal to the twelve o'clock position 3303. Followingalong with the concept, the leading point 1052 may alternatively be at anine o'clock position (FIG. 12A) corresponding to a nine o'clocklocation at the left of the clock, or the leading point 1052 may be at athree o'clock position (not shown) corresponding to a three o'clocklocation at the right of the clock.

Referring to FIG. 29, a pictorial view of a fixation element penetratingtissue in a layered tissue environment is shown in accordance with thepresent disclosure. The layered tissue environment can include amyocardium 2901, e.g., the tissue wall of a right atrial appendage, anda pericardium 2970, e.g., the tissue of a pericardial sac covering theright atrial appendage. In such case, the myocardium 2901 may be 1-2 mmthick, and thus, the leading point 1052 of the fixation element 105 canpenetrate the myocardium 2901 and puncture entirely through the thinwall, as shown.

It has been discovered that the leading point 1052 in the twelve o'clockposition may actually exit the myocardium and grip the pericardium 2970.In some cases, the leading point 1052 can puncture the pericardial sac,or pull the pericardial sac against the myocardium, i.e., pin thepericardial sac to the myocardium. In either case, damage to thepericardium and injury to the patient can result.

Referring to FIG. 30, a pictorial view of a fixation element having aleading point 1052 in a six o'clock position penetrating tissue in alayered tissue environment is shown in accordance with the presentdisclosure. The fixation element 105 can have the leading point 1052 atthe six o'clock position, which can reduce a risk of pinning thepericardial sac. More particularly, the leading point 1052 can penetratemore shallowly, and thus, may not penetrate fully through the myocardium2901. Even if the leading point 1052 does pass through the myocardium2901, a distal trailing edge 3001 of the fixation element 105, which isdistal to the leading point 1052, in the twelve o'clock position canshield the leading point 1052 from the pericardium 2970. For example, incontrast to the leading point 1052, the trailing edge 3001 may not gripthe pericardial sac even when exposed to the pericardium. Accordingly,placement of the leading point 1052 at the six o'clock position canavoid contact between the leading point 1052 and the pericardium 2970and can reduce a likelihood of pericardial pinning and patient injury.Device performance can be therefore be improved by locating the leadingpoint at the six o'clock position.

To address the foregoing issues regarding penetration of the primaryfixation helix, implementations of the present disclosure includeprimary fixation helices in which the sharpened tip is disposed awayfrom the distalmost edge of the helix. For example, in certainimplementations, the sharpened tip may be disposed opposite thedistalmost edge at the end of the helix (at the six o'clock position).By doing so, the likelihood of overpenetration by the helix through acardiac is wall reduced. However, should the helix penetrate through thecardiac wall, the sharpened tip is biased away from the adjacentpericardium because the trailing edge rides over the pericardium 2970,thereby reducing the likelihood that the helix will engage and perforatethe pericardial tissue.

FIG. 2 is an isometric view of a biostimulator 200 in accordance withthe present disclosure. The biostimulator 200 includes a housing 202 anda header assembly 204 coupled thereto. Coupling of the housing 202 tothe header assembly 204 may be accomplished in various ways including,without limitation, one or more of a biocompatible adhesive, a threadedconnection, or ultrasonic welding.

The header assembly 204 generally includes a primary fixation element205 and one or more backstop elements 203. There may be several backstopelements 203, including forward facing and side facing or laterallyextending backstop elements 203, which provide anti-unscrewing features.More specifically, the primary fixation element 205 is a primary helixpointing in a first direction and the backstop elements 203 can includeforward facing anti-unscrewing features 212A, 212B. The forward facinganti-unscrewing features can include several forward facing suturesextending from a forward face of the biostimulator 200 in a seconddirection opposite the first direction.

The primary helix 205 may be substantially formed of any suitablebiocompatible material including, without limitation, one or more ofstainless steel, nickel-titanium alloys (such as Nitinol),nickel-chromium alloys (such as Incoloy®), titanium, and multiphasenickel alloys (such as MP35N® or 35N LT®). In certain implementations,the substrate material of the primary helix 205 may also be conductivesuch that the primary helix 205 may be used as an electrode for sensingand/or pacing of cardiac tissue.

The primary helix 205 is preferably sized to couple the biostimulator200 to cardiac tissue while minimizing damage to the cardiac tissue. Theprimary helix 205 can extend over any number of turns about a helicalaxis to a leading point. In certain implementations, for example, theprimary helix 205 extends from and including 0.25 turns to and including3 turns from the helix mount 206. The primary helix 205 can have a wirediameter from and including 0.003 inches to and including 0.03 inches.The primary helix 205 can have a pitch diameter from and including 0.06inches to and including 0.3 inches. The primary helix 205 can have apitch from and including 0.01 inches to and including 0.05 inches. Otherimplementations of the present disclosure may include multiple fixationhelices in addition to primary helix 205, wherein each helix extends inthe same direction and each helix is adapted to engage cardiac tissue inresponse to rotation of the biostimulator 200. Such multi-heliximplementations may include biostimulators with multifilar helices inwhich multiple wires are conjointly wound or biostimulators includingmultiple offset helices.

Functionality of the sutures 212A, 212B depends, at least in part, ontheir flexibility. Suture flexibility may be controlled by, among otherthings, material selection, and suture dimensions while the overallcounter rotational resistance provided by the forward facing sutures maybe further modified by, among other things, the quantity of suturesemployed and the relative positioning of the sutures. Regardingmaterials, the sutures 212A, 212B may be formed of various flexiblebiocompatible materials including, without limitation, one or more ofpolypropylene, polyethylene, polyester, nylon, polyurethane, silicone,poly(lactic acid) (PLA), poly(glycolic acid) (PGA), polyimide, polyetherether ketone (PEEK), and polycarbonate. Other biocompatible materialsthat may be used to form the non-metallic filaments 212A, 212B includenatural materials. For example, the sutures 212A, 212B can includenatural fibers such as one or more of hair, horse hair, nail, hide,horn, or plant fibers, such as horsetail or thistle. The naturalmaterials can also include sharkskin, which is microporous and has arough surface similar to sandpaper. The rough surface can promotefixation of the secondary fixation element to heart tissue, however,sharkskin may have material characteristics that discourage ingrowth ofcardiac tissue into the secondary fixation element. It is contemplatedthat such anti-ingrowth characteristics can be beneficial in certaincircumstances.

Dimensionally, the length and diameter of the sutures 212A, 212B mayvary depending on the specific configuration of the biostimulator 200,however, in certain implementations the sutures 212A, 212B have a lengthfrom and including 0.003 inches to and including 0.2 inches and adiameter from and including 0.003 inches to and including 0.03 inches.In certain implementations, the flexibility of the sutures 212A, 212B issufficiently high to resist counter rotation caused by general cardiacactivity and movement of the patient but low enough such that removaland/or repositioning of the biostimulator 200 is possible withoutsignificant damage to the cardiac tissue. For example, each of thesutures 212A, 212B may have a stiffness (Young's Modulus) from andincluding 0.5 gigapascals (GPa) to and including 10 GPa. In certainimplementations, the sutures 212A, 212B may include tips that areconfigured to improve engagement with cardiac tissue. For example, thesuture 212A, 212B may be trimmed or otherwise formed to have sharpenedtips.

The header assembly 204 may include multiple components including ahelix mount 206, a cap 208, and a flange 210. Generally, the helix mount206 couples to and retains the primary helix 205 while the cap 208retains each of the several forward facing sutures. The flange 210couples the header assembly 204 to the housing 202 and provides acentral structure to which each of the helix mount 206 and the cap 208are mounted. The flange 210 may further include an electrode 211 thatcontacts tissue when the biostimulator is implanted and through whichelectrical stimulation may be delivered. The example biostimulator 200further includes several laterally extending backstop elements 203. Forexample, backstop elements 203 can include laterally extendingnon-metallic filaments 214A-214C. The laterally extending backstopelements can be lateral sutures 214A-214C (lateral suture 214C beinghidden in FIG. 2). As illustrated in FIG. 2, such lateral sutures may becoupled to and extend from the helix mount 206.

Portions of the header assembly 204 may be coated or filled with abiocompatible epoxy or similar material. For example, in certainimplementations, a gap 250 may be present between the flange 210 and thehelix mount 206 and may be filled with a biocompatible adhesive or epoxysuch as one of NuSil™ medical adhesive 6219 and Hysol® M31-CL. Suchadhesives and epoxies may be used to reinforce coupling betweencomponents of the header assembly 204 and protect the components fromwear and corrosion.

One or more surface modification technologies may also be applied tocontact surfaces of the biostimulator 200. In general, such contactsurfaces may correspond to any component of the biostimulator 200 thatcontacts or otherwise interacts with tissue of the heart when thebiostimulator 200 is implanted. Examples of contact surfaces of thebiostimulator 200 include, without limitation, the face of the cap 208and the exterior surface of the primary helix 205. For example, asurface modification treatment may be applied to the cap 208, in wholeor in part (e.g., only a specific portion of the face 208), to modifythe properties of the cap 208 as compared to the substrate from whichthe cap 208 is substantially formed.

Such technologies may include technologies to, among other things,change one or more of the surface energy, the surface charge, thesurface chemistry, or the surface morphology of the contact surface.Such modifications may be applied to promote a more organized, thinnerfibrous capsule forming about the contact surface when the biostimulator200 is implanted, thereby reducing the effects of such a capsule onpacing thresholds. For example, implantation of the biostimulator 200into the heart may cause the body's natural foreign body response (FBR)to form thick scar tissue around or near a distal end of thebiostimulator 200 or around specific components of the biostimulator200, such as the cap 208 and the primary helix 205. This scar tissue mayultimately impede pacing by the biostimulator 200. By altering theproperties of the contact surface between the biostimulator 200 (or aspecific component thereof) and the heart through the application ofsurface modification technologies, the FBR may be controlled or directedto promote a more predictable tissue reaction. For example, surfacemodification technologies may be applied to promote the formation of arelatively thin and even tissue capsule around the biostimulator 200.Surface modification may also be used to promote improvedsubstrate-to-tissue adhesion, thereby improving fixation of thebiostimulator 200 within the heart tissue.

Various surface modification technologies may be applied to the contactsurface using different techniques. For example, surface energy of thecontact surface may be modified by, among other things, glow dischargeor plasma treatment of the contact surface. As another example, surfacecharge may be modified by material selection or deposition of polymersor other materials that may be electrically charged or conductive ontothe contact surface. Examples of such materials include, withoutlimitation, piezoelectric polymer films and polyvinylidene fluoride(PVDF) films. Surface chemistry may be modified by, among othertechniques, one or more of radiation grafting, protein patterning withsoft lithography or micro-contact printing, and immobilization ofpeptides or proteins in specific micro patterns on the material surface.As yet another example, surface morphology may be modified bytopographical patterning of the contact surface. Such patterningtechniques may include, without limitation, one or more of lasermicromachining and micromolding, such as micromolding usingpolydimethylsiloxane (PDMS).

FIG. 3 is a schematic illustration of a distal face 302 of thebiostimulator 200. For clarity, the biostimulator 200 is illustrated inFIG. 3 as a simple cylinder including the distal face 302 and variousfeatures and components of the biostimulator 200 have been omitted.

The biostimulator 200 defines a longitudinal axis 304 extending throughthe distal face 302. Distributed about the longitudinal axis are forwardfacing sutures 212A, 212B. The remainder of the foregoing discussionwill describe aspects of the suture 212A in detail; however, theforegoing discussion is similarly applicable to the suture 212B.Moreover, while the biostimulator 200 includes two forward facingsutures 212A, 212B, the biostimulator 200 is intended only as an exampleof one implementation of the present disclosure. In otherimplementations, one or greater than two sutures may extend from theforward face 302 of the biostimulator 200. In implementations in whichmore than one sutures extends from the forward face, the multiplesutures may be distributed evenly or unevenly about the forward face302.

The suture 212A extends from a suture origin 310 at a radius r from thelongitudinal axis 304. In certain implementations, r is a distance fromand including 0.03 inches to and including 0.3 inches. For purposes ofestablishing a frame of reference, a first axis 314 extends through thesuture origin 310 parallel to the longitudinal axis 304. A second axis316 extends through the origin perpendicular to the first axis 314 andextends towards the longitudinal axis 304. Finally, a third axis 318extends perpendicular to each of the first axis 314 and the second axis316. The specific location and the orientation of the first axis 314,the second axis 316, and the third axis 318 are intentionally definedrelative to the suture 212A. Accordingly, to the extent a biostimulatorin accordance with this disclosure includes additional sutures, eachsuture of the biostimulator will similarly define a respective frame ofreference.

Forward facing sutures of biostimulators in accordance with thisdisclosure are generally directed in a direction opposite that of aprimary helix (not shown) of the biostimulator 200. In certainimplementations, the sutures may extend from the forward face 302 at apredetermined orientation and have a predetermined length. For example,the suture 212A may extend for a length L from its origin 310 at both ofa first angle α relative to the first axis 314 and a second angle βrelative to the third axis 316. In certain implementations, L is fromand including 0.01 inches to and including 0.3 inches, α is from andincluding 10 degrees to and including 50 degrees, and β is from andincluding 15 degrees to and including 75 degrees.

FIGS. 4A-4D illustrate interaction of the biostimulator 200 and, morespecifically the suture 212A, with a portion of cardiac tissue 402.

FIG. 4A illustrates fixation of the biostimulator 200 to the portion ofcardiac tissue 402. As shown in FIG. 4A, the primary helix 205 isaffixed to the cardiac tissue 402 by rotating the biostimulator in aclockwise direction. As the primary helix 205 engages the cardiac tissue402 and the biostimulator 205 advances, the backstop element 203contacts the tissue. For example, backstop element 203 can be the suture212A, which bends and travels along the cardiac tissue 402 withoutengaging the cardiac tissue 402.

FIG. 4B illustrates anti-rotation behavior of the suture 212A. Afterfixation of the primary helix 205, counter rotation of the biostimulator200 (i.e., rotation in a counter clockwise direction) causes the suture212A to engage the cardiac tissue 402 and, by doing so, the suture 212Aresists further counter rotation and dislodgment of the primary helix205 from the cardiac tissue 402.

The counter rotational resistance provided by the suture 212A isgenerally intended to maintain fixation of the biostimulator 200 to thecardiac tissue 402 during regular cardiac activity. However, in certaininstances, removal and replacement and/or relocation of thebiostimulator 200 may be required. In such instances, the counterrotational resistance provided by the suture 212A may be overcome byapplying additional counter rotational force to the biostimulator 200.As illustrated in FIG. 4C, such counter rotational force may cause thesuture 212A to bend such that the suture 212A is temporarily oriented inthe same direction as the primary helix 205.

As further illustrated in FIG. 4D further counter rotation of the suture212A after the bending illustrated in FIG. 4C causes the suture 212A todisengage from the cardiac tissue 402, allowing the primary helix 205 tobe unscrewed from the cardiac tissue 402. During unscrewing, the sutures212A generally will remain bent until the biostimulator 200 issufficiently detached from the cardiac tissue 402 to allow the suture212A to return to its starting position, as illustrated in FIG. 4E.

FIG. 5 is a top view of the biostimulator 200 of FIG. 2. As shown inFIG. 5, the forward facing backstop elements 203, e.g., sutures 212A,212B, may extend, at least in part, beyond an inner diameter 502 of theprimary helix 205. Such extension may result in sutures extendingbetween adjacent coils of the primary helix 205. For example, the suture212A is shown extending between distal coils of the primary helix 205.Overlap of the sutures 212A, 212B may be used to further modify theanti-rotational resistance provided by the sutures 212A, 212B. Morespecifically, as the biostimulator 200 experiences anti-rotational forceand the sutures 212A, 212B begin to bend, one or more of the sutures212A, 212B may contact the primary helix 205. Such contact may preventadditional bending of the sutures 212A, 212B unless additional counterrotational force is applied.

The lateral backstop features 203, e.g., sutures 214A-214C, are alsoillustrated in FIG. 5. In certain implementations, the lateral sutures214A-214C are side-facing sutures distributed about the helix mount 206and are adapted to resist counter rotational force by engaging tissueadjacent the biostimulator 200. Similar to the forward facing sutures212A, 212B, the lateral sutures 214A-214C may be composed of a flexiblebiocompatible material and may be designed to have a stiffness thatresists unscrewing caused by cardiac and patient activity whilefacilitating removal of the biostimulator 200 without significant tissuedamage. For example, each of the lateral sutures 214A-214C may becomposed of a flexible biocompatible and/or polymeric material such aspolypropylene, polyethylene, polyester, nylon, polyurethane, silicone,PLA, PGA, polyimide, PEEK, and polycarbonate. Other biocompatiblematerials that may be used to form the sutures 214A-214C include naturalmaterials, e.g., natural fibers including one or more of hair, horsehair, nail, hide, horn, or plant fibers, such as horsetail or thistle.The natural materials may include sharkskin. The lateral sutures214A-214C may have a stiffness value (Young's Modulus) from andincluding 0.5 GPa to and including 10 GPa. In certain implementations,the biostimulator 200 may include from and including one to andincluding eight lateral sutures, which may be distributed evenly orunevenly about the helix mount 206. With specific reference to lateralsuture 214A, each of the lateral sutures may be configured to extendfrom the helix mount 206 a predetermined distance d at a predeterminedangle θ relative to a normal 504 extending between the longitudinal axis304 of the biostimulator 200 and an origin 506 of the lateral suture214A. In certain implementations d may be from and including 0.003inches to and including 0.05 inches and θ may be from and including 15degrees to and including 75 degrees. Each of the lateral sutures214A-214C may also have a diameter from and including 0.003 inches toand including 0.03 inches.

FIGS. 6A-6B illustrate endocardial implantation of biostimulators 602A,602B in chambers of a patient heart 600. As shown in FIG. 6A, a firstbiostimulator 602A is implanted within an atrium 604 of the heart 600while a second biostimulator 602B is implanted within a ventricle of theheart 606. Implantation of each of the first and second biostimulators602A, 602B may be achieved, in part, by insertion of the biostimulators602A, 602B endocardially through a guiding catheter. A torqueablecatheter can be used to rotate the respective housings of thebiostimulators 602A, 602B and force the respective primary helices 605A,605B of the biostimulators 602A, 602B into corresponding heart tissue,affixing the primary helices 605A, 605B and corresponding electrodesinto contact with stimulable tissue. Similarly, and as illustrated inFIG. 6B, removal and retrieval of the biostimulators 602A, 602B may alsobe accomplished endocardially through a guiding catheter 608. In theexample of FIG. 6B, the second biostimulator 602B is in the process ofbeing removed from the heart 600. To remove the second biostimulator602B, a torqueable catheter 610 may be inserted into the heart 600through the guiding catheter 608 and coupled to the biostimulator 602B.The torqueable catheter 610 may then be counter rotated to disengage thebiostimulator 602B as described above. A similar process of insertingguide and torque catheters may also be used for epicardial fixation andremoval of biostimulators in accordance with this disclosure.

FIG. 7 is an exploded view of the header assembly 204 of FIG. 2 with theprimary helix 205, forward facing sutures 212, and lateral sutures 214removed for clarity. As illustrated in FIG. 7, the flange 210 mayinclude a central post 702 that extends through each of the helix mount206 and the cap 208 and to which each of the helix mount 206 and the cap208 may be coupled. In certain implementations, the central post 702 mayinclude a threaded section 704 adapted to mate with correspondingthreads of the helix mount 206 to form a threaded connection. In otherimplementations, coupling between the helix mount 206 and the flange210, may be achieved by other methods including, without limitation,adhesives and ultrasonic welding. Accordingly, helix mount 206 can bemounted on housing 102 by attaching helix mount 206 to central post 702and/or flange 210 that is likewise mounted on housing 102. The centralpost 702 may further includes a smooth section 706 capped with theelectrode 211 that is inserted through the helix mount 206.

The cap 208 may be coupled to the helix mount 206 such that the positionand orientation of the forward-facing sutures 212A, 212B (shown in FIG.2) are maintained relative to the helix mount 206 and the primary helix205 (shown in FIG. 2). As shown in FIG. 7, for example, the cap 208defines a keyway 708 into which a corresponding key 710 of the helixmount 206 may be inserted to establish a specific orientation of the cap208 relative to the helix mount 206. Other implementations may includealternative mating features of the cap 208 and the helix mount 206 thatsimilarly insure that the cap 208 is mounted with a particularorientation relative to the helix mount 206. For example, in certainimplementations, the helix mount 206 and the cap 208 may includematching threads, such as single-start threads, adapted to dispose thecap 208 in a predetermined orientation when tightened onto the helixmount 206. The predetermined orientation can include a relative locationbetween a tip of the fixation element 105 and a tip of the one or morebackstop elements 203. For example, the tips of forward-facing sutures212A, 212B can be clear of the leading edge of the primary fixationelement 105 so as to not interfere with the leading edge when itpenetrates tissue. More particularly, the tip of the backstop elements203 can trail the tip of the fixation element 205 by an amount thatensures that, when the distal end of biostimulator 100 is being rotatedagainst tissue, the backstop element 203 does not bend into the pathwayof the fixation element 105 advancement. An example of this is shown inFIG. 2, in which the tip of the forward-facing suture 212A trails theleading edge of fixation element 205 by at least 15 degrees about thelongitudinal axis. For example, the tip of the forward-facing suture212A can trail the leading edge of the fixation element 205 by 30degrees in the counterclockwise direction.

FIGS. 8A-8B are top and bottom views of the cap 208. The cap 208 isgenerally adapted to receive and retain forward facing sutures, such asthe sutures 212A, 212B shown in FIG. 2. To do so, the cap 208 may defineseveral suture bores 802A, 802B within which the sutures 212A, 212B maybe retained and through which the sutures 212A, 212B may extend. Moreparticularly, the non-metallic filaments 203 of biostimulator 100 can beinserted through bores 802A, 802B, or any other bores in cap 208. Eachsuture bore 802A, 802B may be angled to maintain a suture extendingtherethrough at a particular angle. For example, the suture bores 802A,802B may be oriented at the angles α and β previously discussed in thecontext of FIG. 3 such that sutures extending through the sutures bores802A, 802B are maintained at the angles α and β.

In certain implementations, each suture is formed separately andindividually installed in the cap 208. Installation may include, amongother things, applying an adhesive or otherwise fixing the sutures to anunderside of the cap 208 or within a cavity defined by the cap 208. Inother implementations pairs of sutures, such as the sutures 212A and212B of FIG. 2, may also be formed from a single length of suturematerial that is trimmed to length. For example, the cap 208 may includea suture groove 804 extending between suture bores 802A, 802B. Duringmanufacturing, a length of suture material may be inserted into a firstof the suture bores 802A, 802B, run through the suture groove 804 andout a second of the suture bores 802A, 802B. The portion of the threadextending from each of the suture bores 802A, 802B may then be trimmedto length as required. In implementations including additional pairs ofsutures and suture bores, each pair of sutures may similarly be formedfrom a single length of suture material. A single length of suturematerial may also be threaded through multiple pairs of suture bores andrun along corresponding suture grooves such that more than two suturesare formed from the same length of suture material.

FIGS. 9A and 9B are a side elevation view and a cross sectional sideview of the helix mount 206. Helix mount 206 can be mounted on housing102. The helix mount 206 includes a helix mount body 902 defining ahelical groove 904 shaped to receive the primary helix 205. Moreparticularly, helix mount 206 may have a helix mount flange 903 thatextends around the helix mount body 902 in a helical fashion to form athreaded flange form onto which fixation element 205 can be mounted. Forexample, fixation element 205 can include a coiled wire that can bescrewed onto helix mount 206 to secure the fixation element 205 to helixmount 206 and housing 102. Fixation element 205 can be fixed onto helixmount 206 during manufacturing, e.g., by welding, gluing, or otherwisebonding the components together, such that fixation element 205 does notrotate relative to helix mount 206 during operation.

The helix mount 206 may further define several holes or similar cavities906 into which each of the lateral sutures 214A-214C (lateral suture214C is shown in FIG. 5) may be inserted and fixed. For example, a firstend of backstop elements 214A-214C can be positioned within cavities 906and the backstop elements can extend through helix mount flange 904 to asecond end. For example, backstop elements can be non-metallic filamentsthat extend through respective bores in a sidewall 907 of helix mountflange 904 from the first end to the second end. Cavities 906 can befilled with an adhesive or other filler to secure the first end of thenon-metallic filaments within the cavities and fix the backstop elementsto helix mount 206.

Features such as cavities and through-holes that promote tissuein-growth into and through the biostimulator can increase fixation ofthe device to tissue and prevent anti-unscrewing and disengagement ofthe biostimulator from tissue. It should be understood that many of theanti-unscrewing features described herein are configured to preventunintentional detachment of the biostimulator from tissue immediatelyafter implant, but before tissue has had time to grow into the device.In one implementation, the through-holes are angled with an orifice on adistal face of the biostimulator.

The through-holes described herein can be open and free of anyobstructing material, or alternatively, can be filled with afast-dissolving substance, such as mannitol, or with a slowlybioabsorbable material. The advantage of filling the through-holes orcavities prior to implantation of the biostimulator is that iteliminates the risk of trapped air embolism and cavities that can serveas a nidus for bacterial growth.

The anti-unscrewing features described herein are intended to prevent abiostimulator from unintentionally unscrewing or disengaging fromtissue. These features are most critical at the time shortly followingimplantation of the biostimulator (e.g., within 1-3 months ofimplantation). After 1-3 months post-implantation, endothelializationwill have had sufficient time to occur such that the biostimulator isfully encapsulated by tissue. It may be unlikely that a fullyencapsulated biostimulator will inadvertently unscrew itself fromtissue.

Features to prevent unscrewing may be designed to be most effective inthe short time period post-implant (e.g., within the first 1-3 monthsafter implantation). These anti-unscrewing features can therefore bemanufactured out of a bio-absorbable material. Once they are no longerneeded to prevent unscrewing of the biostimulator, they can bioabsorband disappear. Thus, the anti-unscrewing features described herein,e.g., forward facing sutures, may be manufactured out of bioabsorbablematerials to be absorbed by the body after the initial 1-3 month timeperiod post-implant.

A fixation element, such as the primary helix 205 shown in FIG. 2, caninclude cut-outs or indentations along the length of the fixationelement. The cut-outs can include semi-circular cutouts into thefixation element. These cut-outs allow for tissue ingrowth after thefixation element has been inserted into tissue. Although not shown, thecut-outs can include other shapes, including triangular, square,rectangular, etc. shaped cut-outs.

In some implementations of the present disclosure, the electrode may beseparate from the fixation element. In such implementations, anelectrode may be mounted on a flexible arm which extends outwardly fromthe body or housing of the biostimulator. The flexible arm can extendradially outwards from the biostimulator body or housing to provideadditional resistance against tissue in the event that the biostimulatorbegins to unscrew or become dislodged from tissue. The arm may includeadditional anti-unscrewing features, such as through-holes, barbs,teeth, sutures etc., to further prevent anti-unscrewing. In someimplementations, the flexible arm is flexible in only one direction ofrotation (e.g., the direction of rotation that would allow for theleadless biostimulator to unscrew from tissue), and is stiff ornon-flexible in the other direction of rotation.

Referring to FIG. 32A, in an alternative implementation, a perspectiveview of a biostimulator 200 is shown, that includes an active helicalelectrode 211 nestled within the fixation element 205. Moreparticularly, the helices 211 and 205 can both revolve about thelongitudinal axis, however, a helix radius of the fixation element 205may be greater than the helix radius of electrode 211. Moreparticularly, the electrode 211 can be an active helical electrode 211mounted on the housing 102 of biostimulator 200, and the active helicalelectrode 211 can perform a dual pacing and fixation function. Forexample, electrode 211 may include a secondary helix, e.g., an electrodehelix 3203, that extends along an electrode helical axis 3205 about thelongitudinal axis 304. The electrode helical axis 3205 of the activehelical electrode 211 may be radially inward of the helical axis 1001 offixation element 205. In an aspect, the helical axis 1001 of thefixation element 205 and the electrode helical axis 3205 of the activehelical electrode 211 can revolve about the longitudinal axis 304 in asame rotational direction. For example, both helices can revolve in acounterclockwise direction about the longitudinal axis 304.

Active helical electrode 211 may be formed of metallic biocompatiblematerials including, without limitation, platinum iridium. Activehelical electrode 211 may be coated, e.g., with titanium nitride. Thecoating may also include low-polarization coatings, such as such asplatinum, platinum-iridium, iridium, iridium-oxide, titanium-nitride,carbon. Portions of the active helical electrode 211 may be masked toachieve an optimized electrode surface area.

Referring to FIG. 32B, dimensionally, the active helical electrode 211can extend from and including 0.25 turns to and including 3 turns fromthe housing. A wire diameter of the active helical electrode 211 can befrom and including 0.003 inches to and including 0.03 inches, a pitchdiameter from and including 0.02 inches to and including 0.3 inches, anda pitch from and including 0.01 inches to and including 0.1 inches.Active electrode helix 211 may have a different pitch than fixationelement 205, allowing for differing grips on tissue passing through 205during fixation. For example, if active helical electrode 211 pitch was0.1 mm larger than the pitch of fixation element 205, fixation element205 would be compressed by 0.1 mm during the course of fixation, causing205 to compress (or grip) the tissue it has fixated, making dislodgementof the leadless pacemaker 200 less likely, and reducing signal noise.

Referring again to FIG. 32A, the active electrode helix 211 may revolvearound an active agent-eluting payload 3201. For example, the payloadmay be a steroidal plug of material that elutes into the target tissuewhen helix 211 is secured within the target tissue.

Referring again to FIG. 32B, active helical electrode 211 may extend0-2.5 turns, e.g., 2.5 turns beyond the helix mount 206. Active helicalelectrode 211 may be proud of fixation element 205. More particularly, adistal tip of active helical electrode 211 may be distal to a distal tipof fixation element 205. An active helical electrode 211 that is proudof fixation element 205 may allow for sensing of thresholds prior tofixation, reducing the need for repositioning. An example protrusion3207 is shown. The protrusion 3207 may be 0.020 inch, by way of example,which may allow for one full turn of helical electrode 211. A helicalelectrode 211 proud of fixation element 205 may also act as a pilot holefor fixation element 205, allowing the fixation of 205 to remaincentered while the heart is beating. Furthermore, the helical electrode211 proud of fixation element 205 may reduce a likelihood of walking offixation element 205. More particularly, the helical electrode 211 canstabilize fixation element 205 to provide predictable anchoring withinthe target tissue.

In an embodiment, active helical electrode 211 is the same height orlower than fixation element 205 (not shown). A shorter active helicalelectrode 211 may reduce electrode surface area which could prolong abattery life of the biostimulator. A height of active helical electrode211 that is similar or lower than a height of fixation element 205 mayhelp minimize damage to cardiac tissue and the pericardium because theactive helical electrode 211 may fixate to a shallower depth.

The helix of the active electrode may have similar structural featuresas described herein for fixation element 205, for example, both helicesare wound in the same direction so that torsional force on the leadlesspacemaker into tissue will activate both fixation mechanisms at the sametime, thereby adding redundant fixation features and making dislodgementof the leadless pacemaker less likely, and reducing signal noise.

In another embodiment, a leading point of the electrode 211 may be at asix o'clock position to promote shallow penetration of the helix intothe target tissue. By contrast to fixation element 205, however, theelectrode helix 211 may function to deliver electrical impulses to thetarget tissue as described above with respect to electrode 211. Thebiostimulator can be attached to tissue by screwing the fixation element205 and the electrode 211 into the tissue. When the helices are engagedwith the tissue, the electrode 211 is placed in contact with the tissue.Additional anchoring is provided by the increased helical surface areaengaged with the tissue by the helical electrode 211. As describedherein, anti-unscrewing features can be added to prevent thebiostimulator from accidentally dislodging or unscrewing itself fromtissue.

FIGS. 10A-10E illustrate the fixation element 205 of FIG. 2 in furtherdetail. Specifically, FIGS. 10A-10B are side views of the primary helix205 from different angles, FIGS. 10C-10D are detail views of a distalhelix end 1050 of the primary helix 205, and FIG. 10E illustrates a topview of the distal helix end 1050. As shown in FIG. 2, the primary helix205 is generally disposed on a distal end of a biostimulator, such asthe biostimulator 200 of FIG. 2, and is adapted to attach thebiostimulator 200 to a wall of the heart by rotating the biostimulator200 in a screwing direction.

Referring now to FIGS. 10A-10B, fixation element 205 can include a helixextending along a helical axis 1001. For example, the primary helix 205may be a helical spring formed from a wire coil 1002 and extend about ahelix axis 1004, which is generally collinear with a longitudinal axisof the biostimulator 200 (such as longitudinal axis 304, as shown inFIG. 3). In an aspect, the helix is a helical wire, which may be a solidwire or a cut tubing. More particularly, the term wire as used hereinrefers to an elongated element, e.g., a strand, filament, etc., that hasan outer surface extending around helical axis 1001. The outer surface,however, may be formed by cutting, e.g., laser cutting, a spiral cut ina wall of a hypotube to generate a spiral coil. Post-processing, such aselectropolishing, sand blasting, etc., can be used to remove burrs andotherwise smooth the cut edges of the tube-formed helical spring. Thetube-formed helical spring, like the solid wire coil, can have a leadingpoint that may be further processed by grinding and polishing operationsto generate the point morphology described herein. The helical springcan be metallic. Helical axis 1001 of the wire may extend aboutlongitudinal axis 304. The wire coil 1002 can form at least a portion ofthe fixation element 205, which has a cross-section defining an outerperimeter 1006.

In an embodiment, the wire of fixation element 205 extends along helicalaxis to a distal edge 1007. Distal edge 1007 can extend around helicalaxis 1001 to define a helix face 1009 on helical axis 1001. For example,when viewed on end, as in the example of FIGS. 10A-10E, the wire face1009 can be substantially circular. Such may be the case when the wireis a round wire having an outer surface 1011 that is ellipsoidal. Moreparticularly, when the outer surface 1011 extending along helical axis1001 from a proximal end to distal edge 1007 has a circularcross-section, the wire face 1009 has a circular area (when the faceplane is orthogonal to helical axis 1001). By contrast, when theellipsoidal outer surface 1011 is non-circular, e.g., when the wire isan elliptical wire, the distal edge 1007 may be elliptical. Accordingly,the circular perimeter of the wire coil 1002 is merely an example of aperimeter shape that may be used in implementations of the presentdisclosure and primary helices having other shapes and wire coil typesmay be made that conform to the present disclosure. For example, thewire coil 1002 may be generally formed of a biocompatible materialformed into, without limitation, one of a round wire, a flattened orsquare wire, or hypodermic tubing. The wire coil 1002 can be fabricatedfrom metallic biocompatible materials, such as anickel-cobalt-chromium-molybdenum alloy, e.g., MP35N® or 35N LT®,medical grade stainless steel, nitinol or similar metal-basedderivatives having fatigue resistance. The wire coil 1002 can similarlybe formed from machined, cast, or molded plastic.

The distal edge 1007 can be an intersection between the outer surface1011 and the wire face 1009. Accordingly, the distal edge 1007 can be atransition between a wire surface, e.g., a cylindrical surface, and aleading point 1052 of the fixation element 205. The transitional edgecan be angular or rounded. For example, the outer surface of the wirecan be gradually rounded toward the leading point 1052 such that theouter surface transitions continuously to leading point 1052 with noidentifiable angle of the transitioned surface. In such case, the distalface 1009 may be considered any portion of the wire that is radiallyoffset from a center of the wire by a distance that is less than adistance from the center to the outer surface 1009.

In certain implementations, the primary helix 205 may conform topredetermined parameters regarding the shape of the primary helix 205.For example, as shown in FIG. 10A, the primary helix 205 may be formedto have a predetermined pitch 1008 and a predetermined pitch angle 1010.In certain implementations, the pitch 1008 may be from and including0.007 inches to and including 0.060 inches and the pitch angle 1010 maybe from and including 2.5 degrees to and including 20 degrees.

As shown in each of FIGS. 10A-10E, the distal helix tip 1050 mayterminate in the leading point, which may be alternately referred to asa sharpened tip or a tip 1052. The leading point 1052 can be formed by amulti-bevel, e.g., a double-bevel or a triple-bevel, or the outersurface 1009 can be gradually reduced to the leading point 1052 withoutthe use of bevel surfaces.

Leading point 1052 can be a distalmost point on the wire in thedirection of helical axis 1001. Accordingly, the leading point 1052 canbe on distal face 1009. It has been discovered that a location of theleading point 1052 on distal face 1009 can affect a penetrating abilityof fixation element 205. More particularly, when deployed into tissue,fixation element 205 may dig more shallowly into the tissue when leadingpoint 1052 is more proximal relative to housing 102. By way of example,when deployed into heart tissue, a leading point 1052 positioned at asix o'clock position (most proximal relative to housing 102 and ondistal edge 1007 as shown in FIG. 10C) may tend to penetrate less deeplythan a leading point 1052 positioned at a twelve o'clock position (mostdistal relative to housing 102 as shown in FIG. 11A). Accordingly, whendeployed into thinner tissue walls, such as atrial walls, the “sixo'clock” point configuration may reduce a likelihood of penetratingfully through the tissue wall and pinning external structures, such asthe pericardium.

In an aspect, leading point 1052 can be located at a more or less distallocation to control a depth of penetration. For example, referring toFIG. 10C, leading point 1052 can be between a transverse plane 1051 andthe housing 102 to cause fixation element 105 to penetrate less deeplyinto target tissue. More particularly, leading point 1052 can beproximal to the transverse plane 1051. By contrast, leading point 1052can be on an opposite side of transverse plane 1051 from housing 102,e.g., distal to the transverse plane, to cause fixation element 105 topenetrate more deeply into the target tissue.

Transverse plane 1052 can be orthogonal to longitudinal axis 304 (orhelix axis 1004) and may extend along a median line of wire face 1009.More particularly, transverse plane 1051 can intersect a center 1053 ofwire face 1009. Accordingly, transverse plane can define alongitudinally located separator between a portion of wire face 1009that is proximalmost (the surface area that is between transverse plane1051 and housing 102) and a portion of wire face 1009 that is distalmost(the surface area that is on an opposite side of transverse plane 1051from housing 102). When leading point 1052 is located at a positionwithin the proximalmost portion of wire face 1009, fixation element 205can penetrate more shallowly, and vice versa. Several differentpositions are described in more detail below, however, it will beappreciated that leading point 1052 can be located anywhere on wire face1009 to control penetration depth in accordance with the principlesoutlined above.

In an embodiment, as shown in FIGS. 10A-10D, tip 1052 may be disposed onthe outer perimeter 1006, e.g., distal edge 1007, of the wire coil 1002.For example, the tip 1052 may be disposed in a proximal or “six o'clock”position that reduces penetration depth of the tip 1052 during screwinginto cardiac tissue, thereby reducing the likelihood of overpenetrationand corresponding trauma. The six o'clock position can be on alongitudinal plane 1059. The longitudinal plane 1059 may intersecttransverse plane 1051 at center 1053. More particularly, longitudinalplane 1059 can be orthogonal to transverse plane 1051 and parallel tolongitudinal axis 304 such that the intersection of the planes at thecenter divides wire face 1009 into quadrants when viewed on end. In anembodiment, leading point 1052 is on the longitudinal plane 1059. Forexample, leading point 1052 can be on longitudinal plane 1059 proximalto transverse plane 1051 (FIG. 10C) or distal to transverse plane 1051(FIG. 11C). In an embodiment, leading point 1052 is at the six o'clockposition, which is on distal edge 1007 at the intersection of distaledge 1007 and longitudinal plane 1059 (FIG. 10C).

In an embodiment, wire face 1009 includes a plurality of bevel facesconverging at leading point 1052. For example, referring to FIG. 10C, afirst bevel face 1090A and a second bevel face 1090B form separateportions of wire face 1009. Referring to FIG. 10D, the bevel faces canintersect along a leading edge 1092. The leading edge 1092 can be anedge 1054 extending between the tip 1052 and base 1056. Moreparticularly, leading edge 1092 can extend from leading point 1052 tobase 1056 on distal edge 1007 along wire face 1009. Base 1056 can be onan opposite side of transverse plane 1051 from housing 102.

The general position and orientation of the tip 1052 may be described interms of the relative angle of the edge 1054. For example, each of FIGS.10A-10D include a vector 1058 distally extending from the base 1056parallel to the helix axis 1004 such that a plane 1060 is defined by thehelix axis 1004 and the vector 1058. The orientation of the tip 1052 maythen be described in terms of an angle θ (shown in FIG. 10C)corresponding to an angle away from the vector 1058 along the plane1060. The angle θ may be from and including 0 degrees to and including180 degrees, the latter of which corresponds to the “six o'clock”position illustrated in FIGS. 10A-10D. A second angle ψ may specify theforward extension of the tip 1052. More specifically, the second angle ψmay correspond to the angle at which the edge extends relative to theplane 1060. In certain implementations, ψ may be from and including 10degrees to and including 60 degrees. The distal helix tip 1050 may alsohave a predetermined tip length 1070, which generally corresponds to thelongitudinal distance between the base 1060 and the tip 1052. Forexample, in certain implementations, the tip length 1070 may be from andincluding 0.002 inches to and including 0.03 inches.

FIGS. 11A-14C illustrate fixation element embodiments having variationsof leading point configurations. For example, leading point 1152, 1252,1352, 1452 is located at different positions or is formed by theconvergence of different numbers of bevel faces as compared to FIGS.10A-10D. It will be appreciated that, even when the reference geometryand terminology used above, e.g., transverse plane 1051, longitudinalplane 1059, etc., is not explicitly used in the description below, theconfigurations are nonetheless implicitly described with such terms.

FIGS. 11A-11C are schematic illustrations of a primary helix 1100according to the present disclosure. Primary helix 1100 is a fixationelement 1100 having a leading point 1152 on distal edge 1007 distal totransverse plane 1051. More particularly, leading point 1152 is at thetwelve o'clock position on longitudinal plane 1059. FIG. 11A is a sideview of the primary helix 1100 and FIGS. 11B-11C are detail views of adistal helix end 1150 of the primary helix 1100. The primary helix 1100may be formed from a wire coil 1102 and extend about a helix axis 1104.The wire coil 1102 forming the primary helix 1100 includes an outerperimeter 1106.

As shown in each of FIGS. 11A-11C, the distal helix tip 1150 mayterminate in a sharpened tip 1152 formed by a double-bevel such that thetip 1152 is disposed on the outer perimeter 1106 of the wire coil 1102.In contrast to the primary helix 205 illustrated in FIGS. 10A-D in whichthe tip 1052 was disposed in a proximal or “six o'clock” position, thetip 1152 of the primary helix 1100 is position at a distal or “twelveo'clock” position. More specifically, the double bevel forming the tip1152 results in an edge 1154 extending between the tip 1152 and base1156. A vector 1158 distally extends from the base 1156 parallel to thehelix axis 1104 such that a plane 1160 is defined by the helix axis 1104and the vector 1158. In the “twelve o'clock” position, the angle θbetween the vector 1158 and the base 1156 along the plane 1160 issubstantially zero. The “twelve o'clock” position of the tip 1152generally corresponds to a more aggressive tip as compared to theprimary helix 205 of FIGS. 10A-10D. Accordingly, such a primary helix ismore suitable for implantation in relatively thick cardiac tissue orimplantation in locations in which overpenetration will not result inunnecessary trauma, e.g., a ventricle. As shown in FIG. 11B and similarto the tip 1052 of the primary helix 1000, the forward extension of thetip 1152 may be defined by a second angle ψ. More specifically, thesecond angle ψ may correspond to the angle at which the edge extendsrelative to the plane 1160. In certain implementations, ψ may be fromand including 10 degrees to and including 60 degrees. The distal helixtip 1150 may also have a predetermined tip length 1170, which generallycorresponds to the longitudinal distance between the base 1160 and thetip 1152. For example, in certain implementations, the tip length 1170may be from and including 0.002 inches to and including 0.02 inches.

FIGS. 12A-12C are schematic illustrations of a primary helix 1200according to the present disclosure. Primary helix 1200 is a fixationelement 1200 having a leading point 1252 on distal edge 1007 alongtransverse plane 1051. More particularly, leading point 1252 is at thenine o'clock position on transverse plane 1051. Wire face 1209 has adouble-bevel design such that two bevel faces converge at leading point1252. FIG. 12A is a side view of the primary helix 1200 and FIGS.12B-12C are detail views of a distal helix end 1250 of the primary helix1200. The primary helix 1200 may be formed from a wire coil 1202 andextend about a helix axis 1204. The wire coil 1202 forming the primaryhelix 1205 includes an outer perimeter 1206.

As shown in each of FIGS. 12A-12C, the distal helix tip 1250 mayterminate in a sharpened tip 1252 formed by a double-bevel such that thetip 1252 is disposed on the outer perimeter 1206 of the wire coil 1202.In contrast to the primary fixation helix 205 illustrated in FIGS. 10A-Din which the tip 1052 was disposed in a proximal or “six o'clock”position, the tip 1252 of the primary helix 1200 is positioned at anexternally lateral or “9 o'clock” position. More specifically, thedouble bevel forming the tip 1252 results in an edge 1254 extendingbetween the tip 1252 and base 1256. A vector 1258 distally extends fromthe base 1256 parallel to the helix axis 1204 such that a plane 1260 isdefined by the helix axis 1204 and the vector 1258. In the “9 o'clock”position, the angle θ between the vector 1258 and the base 1256 alongthe plane 1260 is approximately 90 degrees. The “9 o'clock” position ofthe tip 1252 generally corresponds to a middle ground between theconservative “six o'clock” placement of the tip 1052 illustrated inFIGS. 10A-D and the aggressive “twelve o'clock” placement of the tip1152 illustrated in FIGS. 11A-C.

As described above, tip positions have unique benefits in terms ofdevice efficacy, e.g., anchoring, and safety. For example, tip positioncan shield the leading edge of the tip from pericardial pinning toreduce a likelihood of negative effects on long-term pacing performance.Accordingly, biostimulator 100 can be fabricated with a tip positionthat is specific to an intended implant location. By way of example,biostimulator 100 intended for implantation within a ventricle may bemanufactured with a twelve o'clock tip position. By contrast,biostimulator 100 intended for implantation within an atrium may bemanufactured with a six o'clock tip position.

As shown in FIGS. 10A-12C, the angle θ is measured away from the helixaxis. However, in other implementations, the angle θ may instead bemeasured towards the helix axis. For example, an implementation in whichthe angle θ is approximately 90 degrees may correspond to a “3 o'clock”placement of the tip 1052 (i.e., opposite the placement of the tip 1252illustrated in FIGS. 12A-C). Accordingly, to the extent this disclosurerefers to the angle θ and provides ranges of the value of θ, such valuesmay be based on the angle being measured towards or away from the helixaxis.

In certain implementations of the present disclosure, the tip of theprimary fixation helix may instead be provided with respect to anenvelope defined by the primary helix. For example, FIG. 13A is aschematic illustration of a primary helix 1300 including a distal helixend 1350 with a sharpened tip 1352 and FIGS. 13B-13C are side and frontviews, respectively, of the distal helix end 1350.

As shown in FIGS. 13A-13C, primary helix 1300 can be a fixation element1300 having a leading point 1352 on wire face 1009 inward from distaledge 1306 and distal to transverse plane 1051. More particularly,leading point 1152 is along longitudinal plane 1059 in the direction ofthe twelve o'clock position. Wire face 1009 has a triple-bevel designsuch that three bevel faces converge at leading point 1252. The distalhelix end 1350 is formed from a coil wire 1302 and includes a taperedportion 1354 terminating in the sharpened tip 1352. In contrast to theprimary helices of FIGS. 10A-12C in which the respective sharpened tipswere disposed about the perimeter, e.g., on the distal edge 1007, of thecoil wire, the tip 1352 of the primary helix 1300 is disposed within aprojected volume 1356 defined by an untapered portion 1355 of the coilwire 1302. The projected volume 1356 corresponds to a volume that wouldbe occupied by the untapered portion 1355 if the untapered portion 1355were to be extended beyond the tapered portion 1354. Accordingly, thetapered portion 1354 extends into and terminates at the tip 1352 withinthe projected volume 1356.

As shown in FIG. 13B, the position of the tip 1352 may be defined by afirst distance 1358 from the end of the untapered portion 1354 and asecond distance 1360 from a first axis 1357 defined by and extendinglongitudinally through the coil wire 1302. The second distance 1360 maybe defined along a second axis 1359 that perpendicularly intersects afirst axis 1357 and that is parallel to helix axis 1304 (shown in FIG.13A). So, for example, in implementations in which the second distance1360 is zero, the tip 1352 is disposed along the first axis 1357. Inother implementations in which the second distance 1360 is non-zero, thetip 1352 is displaced relative to the first axis 1357. For example, thesecond distance 1360 of the primary helix 1300 of FIGS. 13A-13C is anon-zero value in the distal direction such that the tip 1352 isdistally displaced relative to the first axis 1357. In otherimplementations, the second distance 1360 may correspond to a distancealong the second axis 1359 in a substantially proximal direction.

In certain implementations, the first distance 1358 and the seconddistance 1360 may fall within predetermined ranges. For example, thefirst distance 1358 may be from and including 0.002 inches to andincluding 0.03 inches. Similarly, the second distance 1360 may be fromand including 0.001 inches to and including 0.009 inches and, in oneimplementation, may be 0.004 inches. In one specific implementation, thefirst distance may be 0.01 inches and the second distance may be 0.004inches.

FIG. 14A is a schematic illustration of a primary helix 1400 including adistal helix end 1450 with a sharpened tip 1452 and FIGS. 14B-14C areside and front views, respectively, of the distal helix end 1450.

As shown in FIGS. 14A-14C, primary helix 1400 can be a fixation element1400 having a leading point 1452 on wire face 1009 inward from distaledge 1406 and proximal to transverse plane 1051. More particularly,leading point 1452 is in a lower-left quadrant defined by theintersection of transverse plane 1051 and longitudinal plane 1059. Wireface 1009 has a triple-bevel design such that three bevel faces convergeat leading point 1452. The distal helix end 1450 is formed from a coilwire 1402 and includes a tapered portion 1454 terminating in thesharpened tip 1452. Similar to the primary helix 1300 of FIGS. 13A-13C,the tip 1452 of the primary helix 1400 is disposed within a projectedvolume 1456 defined by an untapered portion 1455 of the coil wire 1402.

As shown in FIG. 14B, the position of the tip 1452 may be defined by afirst distance 1458 from the end of the untapered portion 1454 and asecond distance 1460 from a first axis 1457 defined by and extendinglongitudinally through the coil wire 1402. The second distance 1460 maybe defined along a second axis 1459 that perpendicularly intersects thefirst axis 1456 and that is parallel to a helix axis 1404 (shown in FIG.14A). In contrast to the primary helix 1300 of FIGS. 13A-13C, theposition of the tip 1452 of the primary helix 1400 of FIGS. 14A-14C isfurther defined by an angle α relative to the second axis 1459.Accordingly, the tip 1452 is offset from each of the first axis 1458 andthe second axis 1459.

In certain implementations, the first distance 1458 and the seconddistance 1460 may fall within predetermined ranges. For example, thefirst distance 1458 may be from and including 0.002 inches to andincluding 0.02 inches and the second distance 1460 may be from andincluding 0 inches to and including 0.01 inches. Similarly, the angle αmay be in a range from and including 90 degrees to and including 160degrees. In one specific implementation, for example, the first distancemay be 0.008 inches, the second distance may be 0.002 inches and theangle α may be 120 degrees.

FIG. 15 is an isometric view of the biostimulator 200 of FIG. 2illustrating a suture arrangement. As previously discussed in thecontext of FIG. 2, the biostimulator 200 includes a housing 202 and aheader assembly 204 coupled thereto. The header assembly 204 generallyincludes a primary fixation element 205, which in FIG. 15 is a primaryhelix 205, that extends about a longitudinal axis 304 defined by thebiostimulator 200. The primary helix 205 may be formed of similarmaterials and have similar dimensional characteristics as previouslydiscussed in the context of FIG. 2. As illustrated in FIG. 14, theprimary helix 205 is coupled to the header assembly 204 such that theprimary helix 205 extends distally beyond a distal face 207 of theheader assembly 204. The distal face 207 may correspond to a distal endof the cap 208 of the header assembly 204.

In certain embodiments, the header assembly 204 further includesbackstop features 212A, 212B that extend from the distal face 207. Theanti-unscrewing features 212A, 212B of FIG. 15, for example, areflexible sutures 212A, 212B that extend from the distal face 207 of thebiostimulator 200. In certain implementations, the sutures 212A, 212Bmay also extend at an angle opposite the direction of the primary helix205, as discussed below in more detail. As previously discussed in thecontext of FIG. 2, the sutures 212A, 212B may be formed of variousflexible biocompatible materials including, without limitation, one ormore of polypropylene, polyethylene, polyester, nylon, polyurethane,silicone, poly(lactic acid) (PLA), poly(glycolic acid) (PGA), polyimide,polyether ether ketone (PEEK), and polycarbonate. Backstop features212A, 212B can also be formed from the natural materials describedabove. As illustrated in FIG. 15, the biostimulator 200 may also includelateral cavities, such as lateral cavity 213, within which additionalsutures may be disposed. Lateral cavities 213 can be bores in thesidewall of helix mount 206 through which backstop elements 203, e.g.,non-metallic filaments, can be inserted and secured. Such sutures areillustrated, for example, as sutures 214A, 214B in FIG. 2.

FIG. 16 is a distal view of the biostimulator 200 of FIG. 15. Asillustrated, the primary helix 205 generally extends about thelongitudinal axis 304 at a helix radius 215. The helix radius 215generally corresponds to the pitch radius of the primary helix 205. Incontrast, the sutures 212A, 212B originate from the distal face 207 at afirst suture radius 217A and a second suture radius 217B, respectively.As illustrated in FIG. 16, the suture radii 217A, 217B may differ foreach suture; however, in general, each suture radius may be less thanthe helix radius 215 such that the sutures 212A, 212B extend, at leastpartially, through a volume 209 defined by the primary helix 205. Asillustrated in FIG. 16, the sutures 212A, 212B may also extend alongrespective filament axes 1602 to respective filament tips 1604. Thebackstop elements 212A, 212B can extend beyond the helix radius 215 suchthat the sutures 212A, 212B terminate outside of the volume 209. Whenextending beyond the helix radius 215, each of the sutures 212A, 212Bmay extend between adjacent turns of the primary helix 205, as discussedbelow in the context of FIGS. 18-19 in more detail.

FIG. 17 is a distal view of the biostimulator 200 of FIG. 15 with theprimary helix 205 removed for clarity. As indicated in FIG. 17, each ofthe sutures 212A, 212B may be “swept” such that they extend, at least inpart, in a direction opposite the rotational direction of the primaryhelix 205. Such counter directional biasing of the sutures 212A, 212Bmay, in certain implementations, improve the resistance provided by thesutures 212A, 212B to unscrewing of the primary helix 205. In FIG. 17,for example, each of the sutures 212A, 212B extend at an angle φrelative to respective lines 221A, 221B extending from the longitudinalaxis 304 to their respective origins. In certain implementations, theangle φ may be from and including 15 degrees to and including 75degrees.

FIGS. 18-19 are side elevation views of the biostimulator 200 intendedto illustrate the relative longitudinal placement of the sutures 212A,212B and the primary helix 205. As previously noted, in certainimplementations, the sutures 212A, 212B may be arranged such that theyextend between adjacent turns of the primary helix 205. In other words,the primary helix 205 is configured to distally extend beyond each ofthe sutures 212A, 212B. In such implementations, the primary helix 205must be screwed in a predetermined number of turns before the sutures212A, 212B are able to engage tissue adjacent the implantation locationof the primary helix 205. In certain implementations, the primary helix205 may extend from and including an eighth of a turn to and includingtwo full turns beyond the sutures 212A, 212B. As illustrated in FIG. 18,for example, the primary helix 205 extends approximately one half turnbeyond the suture 212A. As illustrated in FIG. 19, the primary helixextends approximately one full turn beyond the suture 212B. Asillustrated in the embodiments of FIGS. 18 and 19, if the helix isengaged one to one and a half turns into tissue, then the placement ofthe sutures will be within the tissue, guaranteeing activation of thebackstop elements.

The sutures 212A, 212B are illustrated in FIGS. 15-19 as extendingsubstantially perpendicular to the longitudinal axis 304 of thebiostimulator 200. In other implementations, such as discussed in FIGS.2-9B, biostimulators in accordance with the present disclosure mayinclude sutures that extend distally along filament axes to filamenttips, at least in part. The biostimulator 200 of FIG. 2, for example,includes sutures 212A, 212B that extend in a partially distal directionfrom the cap 208. As discussed in the context of FIG. 3, such suturesmay extend from and including 15 degrees to and including 75 degreesrelative to a longitudinal axis 304 of the biostimulator 200. Suchsutures 212A, 212B may also be “swept” at an angle such that they aredirected, in part, counter to the direction of the primary helix 205.Such an angle may, in certain implementations, be from and including 15degrees to and including 75 degrees. Notably and similar to theimplementation of FIGS. 15-19, FIG. 2 illustrates the suture 212B asextending between adjacent turns of the primary helix 205 such that theprimary helix 205 extends approximately one eighth of a turn beyond thesuture 212B.

FIGS. 20-22 illustrate an example biostimulator 2000 incorporatingvarious concepts previously described in this disclosure. Morespecifically, FIG. 20 is a first isometric view of the biostimulator2000, FIGS. 21A-B are lateral side views of the biostimulator 2000focusing on a distal end of the biostimulator 2000 and FIG. 22 is adistal view of the biostimulator 2000.

Although other configurations are possible, the biostimulator 2000illustrates one specific configuration found to have advantageousperformance characteristics during testing. Accordingly, to the extentspecific characteristics of the biostimulator 2000 and its componentsare provided below, such discussion is intended to be non-limiting andto provide merely one example of a biostimulator in accordance with thisdisclosure.

Referring to FIGS. 20-21, the biostimulator 2000 includes a housing 2002and a header assembly 2004 coupled thereto. The header assembly 2004generally includes a primary fixation element 2005 in the form of aprimary helix 2005, that extends about a longitudinal axis 2001 definedby of the biostimulator 2000. The primary helix 2005 may be formed ofsimilar materials and have similar dimensional characteristics aspreviously discussed in the context of FIG. 2. As illustrated in FIG.20, the primary helix 2005 is coupled to the header assembly 2004 suchthat the primary helix 2005 extends distally beyond a distal face 2007of the header assembly 2004. The distal face 2007 may correspond to adistal end of the cap 2008 of the header assembly 2004. Distal face 2007may not be perpendicular to axis 2001, however.

The header assembly 2004 further includes distal lateral sutures 2012A,2012B that extend laterally from the distal face 2007 and proximallateral sutures 2013A-2013C that extend laterally from a helix mount2006 of the header assembly 2004 proximal to the distal lateral sutures2012A, 2012B. In general, each of the distal lateral sutures 2012A,2012B and the proximal lateral sutures 2013A-2013C (proximal lateralsuture 2013C is shown in FIG. 23) extend laterally at an angle oppositethe direction of the primary helix 2005.

The primary helix 2005 includes a single, helically wound wire that issized to couple the biostimulator 2000 to cardiac tissue whileminimizing damage to the cardiac tissue. As most clearly visible inFIGS. 20 and 21, the primary helix 2005 extends approximately 1.5 turnsfrom a distal end 2050 of the helix mount 2006. In general, extendingapproximately 1.5 turns from the distal end 2050 of the helix mount 2006provides a balance between the ease with which the primary helix 2005may be made to engage the wall of the heart and the strength of suchengagement. The primary helix 2005 is also formed of a wire having adiameter of approximately 0.016 inches and a pitch of approximately0.032 inches. Regarding material, the primary helix 2005 is formed oftype-302 stainless steel, which has a yield/ultimate tensile strength ofapproximately 350 kilopound per square inch (ksi). Other biocompatiblematerials having similar properties may also be used in otherimplementations. For example, in one implementation, instead of type-302stainless steel, an alloy such as 35N LT® may be used for the primaryhelix 2005.

Similar to the primary helix 1200 illustrated in FIGS. 12A-12C, theprimary helix 2005 can include a sharpened tip 2052 that may be formed,for example, by a double-bevel and that is disposed on an outerperimeter of the wire forming the primary helix 2005. As shown, thesharpened tip 1252 is disposed at approximately a “9 o'clock” position,however, that position can be different as described above. In otherwords, the sharpened tip 2052 is disposed on a lateral extent of theprimary helix 2005 relative to the longitudinal axis 2001.

As illustrated in each of FIGS. 20-22, the biostimulator 2000 includes apair of distal lateral sutures 2012A, 2012B that extend laterally from acap 2008 disposed at a distal end of the biostimulator 2000. It is notedthat, although FIGS. 20-22 illustrate fixation element 2005 extending 2turns beyond helix mount 2006, the helical anchor may extend by anotheramount, e.g., between 1.5 to 2.5 turns, beyond the helix mount. Forexample, the fixation element can extend more than 2.5 turns from thehelix mount, and thus, the illustration is provided by way of exampleand not limitation. The distal lateral sutures 2012A, 2012B aredistributed about the cap 2008 such that they are disposed approximately180 degrees apart from each other. The distal lateral sutures 2012A,2012B are also disposed at locations that are offset approximately 90degrees and 270 degrees, respectively, from the tip 2052 of the primaryhelix 2005.

In the specific example of the biostimulator 2000, the distal lateralsutures 2012A, 2012B are #3-0 sutures having a diameter ranging from andincluding approximately 0.2 millimeters to and including approximately0.25 millimeters and are formed from polypropylene. As illustrated inFIG. 22, each of the distal lateral sutures 2012A, 2012B extendslaterally from the distal cap 2008 such that a filament tip 2015A, 2015Bof each suture 2012A, 2012B extends approximately 0.045 inches (+/−0.005inches) from the cap 2008. In other implementations, the sutures 2012A,2012B may extend from the distal cap 2008 by a distance from andincluding 0.020 inches to and including 0.080 inches. The suture 2012Aextends at an angle of approximately 30 degrees relative to a firstdistal radial line 2009A extending from the longitudinal axis 2001through an origin 2011A of the suture 2012A, the origin 2011Acorresponding to a location of the cap 2008 from which the suture 2012Aemerges. Similarly, the suture 2012B also extends at an angle ofapproximately 30 degrees relative to a second distal radial line 2009Bextending from the longitudinal axis 2001 through an origin 2011B of thesuture 2012B, the origin 2011B corresponding to a location of the cap2008 from which the suture 2012B emerges. In other implementations, thedistal sutures 2012A, 2012B may extend relative to their respectivedistal radial lines 2009A, 2009B at an angle from and including 15degrees to and including 75 degrees.

In an embodiment, each filament tip can be shaped to have a filamentface along the respective filament axis 1602, which is at an angle tothe filament axis. For example, as shown, each tip 2015A, 2015B can becut or trimmed at approximately a 30 degree angle relative to therespective filament axes.

As previously discussed, in addition to the distal lateral sutures2012A, 2012B, the leadless biostimulator 2000 further includes a set ofproximal lateral sutures 2013A-2013C extending from the helix mount2006. The proximal lateral sutures 2013A-2013C are evenly distributedabout the helix mount 2006 such that they are disposed 120 degrees apartfrom each other. The proximal lateral sutures 2013A-2013C are alsodisposed at locations that are offset approximately 60 degrees, 180degrees, and 300 degrees, respectively, from the tip 2052 of the primaryhelix 2005. In another example implementation, the proximal lateralsutures 2013A-2013C may instead be disposed at locations that are offsetapproximately 45 degrees, 165 degrees, and 285 degrees, respectively,from the tip 2052 of the primary helix 2005. In still otherimplementations, the proximal lateral sutures 2013A-2013C may instead bedisposed at locations that are offset from and including 30 degrees toand including 70 degrees, from and including 150 degrees to andincluding 190 degrees, and from and including 270 degrees to andincluding 320 degrees, respectively, from the tip 2052 of the primaryhelix 2005.

In an embodiment, the backstop elements, e.g., the non-metallicfilaments 2013A-2013C, can extend through respective bores in thesidewall of helix mount. Furthermore, the bores can be at differentlongitudinal positions relative to longitudinal axis 304 such that thebackstop elements 203 are longitudinally offset from each other. Asillustrated in FIG. 21A, the proximal lateral sutures 2013A-2013C may belongitudinally offset from the distal lateral sutures 2012A, 2012B by adistance 2021 of approximately 0.010 inches. In other implementations,the distance 2021 may be from and including 0.005 inches to andincluding 0.050 inches. Also, as illustrated in FIG. 21, thelongitudinal distance between the distal lateral sutures 2012A, 2012Band the proximal lateral sutures 2013A-2013C may vary for each of theproximal lateral sutures 2013A-2013C. For example, the helix mount 2006may have a helical shape and the proximal lateral sutures 2013A-2013Cmay extend at various points along the helical shape such that each ofthe proximal lateral sutures 2013A-2013C is disposed at a differentlongitudinal location.

In the specific example of the biostimulator 2000, the proximal lateralsutures 2013A-2013C are #4-0 sutures having a diameter ranging betweenapproximately 0.15 mm and 0.2 mm and are formed from nylon. Asillustrated in FIG. 22, each of the proximal lateral sutures 2013A-2013Cextends laterally from the helix mount 2006 such that a tip 2017A-2017Cof each suture 2013A-2013C extends approximately 0.020 inches (+/−0.005inches) from the helix mount 2006. In other implementations, the sutures2013A-2013C may extend from the helix mount 2006 by a distance from andincluding 0.010 inches to and including 0.030 inches. The suture 2013Aextends at an angle of approximately 30 degrees relative to a firstproximal radial line 2023A extending from the longitudinal axis 2001through an origin 2019A of the suture 2013A, the origin 2019Acorresponding to a location of the helix mount 2006 from which thesuture 2013A emerges. The suture 2013B also extends at an angle ofapproximately 30 degrees relative to a second proximal radial line 2023Bextending from the longitudinal axis 2001 through an origin 2019B of thesuture 2013B, the origin 2019B corresponding to a location of helixmount 2006 from which the suture 2012B emerges. Finally, the suture2013C extends at an angle of approximately 30 degrees relative to athird proximal radial line 2023C extending from the longitudinal axis2001 through an origin 2019C of the suture 2013C, the origin 2019Ccorresponding to a location of helix mount 2006 from which the suture2013C emerges. In other implementations, the proximal sutures2013A-2013C may extend relative to their respective proximal radiallines 2023A-2023C at an angle from and including 15 degrees to andincluding 75 degrees.

In an embodiment, each filament tip can be shaped to have a filamentface along the respective filament axis 1602, which is at an angle tothe filament axis. For example, filament tips 2015A, 2015B can be cut ortrimmed at approximately a 30 degree angle relative to the respectivefilament axes 1602.

Referring to FIG. 21B, in an embodiment, the several backstop elements2113A, B and 2012A of biostimulator 2100 can include a pinch point 2110between the wire of fixation element 2105 and a surface of the helixmount 2106. For example, fixation element 2105 can spiral proximallyfrom the leading point until the wire abuts helix mount flange 2160.More particularly, an upper surface of helix mount flange 2160 canappose the outer surface of the wire at a pinch point 2110. Pinch point2110 is between the wire and the distal end of helix mount flange 2160such that, as fixation element 2005 screws into tissue, the capturedtissue will become wedged between the wire and the helix mount flange atthe pinch point. When tissue is wedged at the pinch point 2110, theclamping force applied to the tissue causes biostimulator 100 to resistback-out forces applied by the dynamic operating environment.Accordingly, pinch point 2110 acts as another fixation mechanism thatresists the backward movement of fixation element 2005.

Referring to FIG. 26, a graphical view of a back-out torque of abiostimulator is shown in accordance with an embodiment. The graph canplot back-out torque against rotation angle, where the rotation anglecorresponds to different tissue engagement events. For example, asfixation element 205 engages and screws into tissue, the back-out torquerequired to dislodge biostimulator can be zero when the leading pointinitially punctures the target tissue to a first value when a firstbackstop element 203 engages the target tissue at rotation angle 1602.The required back-out torque can experience a step increase at rotationangle 1602 because dislodgment requires not only overcoming the frictionof the wire within the target tissue, but also the resistance torqueprovided by the first backstop element. As more backstop elements engagethe target tissue, the required back-out torque can increase. Forexample, rotation angle 1602 can correspond to engagement of a forwardfacing non-metallic filament with the target tissue, and rotation angle1604 can correspond to engagement of a side facing non-metallic filamentwith the target tissue. At rotation angle 1604, the back-out torque canexperience another step increase because the additional backstopelement(s) further resist back-out.

In an embodiment, the removal torque increases further when fixationelement 205 engages tissue and the tissue is wedged in the pinch point2110. For example, when the tissue is at the pinch point, tissueshearing may occur. The tissue shearing can cause localized scarring,which can increase adhesion between the target tissue and the wire offixation element 205. The increased adhesion can in turn cause asubstantial increase in removal torque required to dislodgebiostimulator 100. For example, the biostimulator can have a firstremoval torque when fixation element is engaged in tissue and the tissueis not at the pinch point 2110, and biostimulator 100 can have a second,higher, removal torque when fixation element is engaged in the tissueand the tissue is at the pinch point 2110. The latter case is shown atrotation angle 1606. In an embodiment, a rate of increase in back-outtorque ramps upward at position 1606 because more shearing occurs as thepinch point advances over the tissue, and the scarring provides anincrease in adhesion that has a relatively higher resistance toback-out. By way of example, the second removal torque when tissue is atpinch point can be at least 10% higher than the first removal torquewhen tissue is not at pinch point.

During testing, a biostimulator having characteristics of thebiostimulator 2000 exhibit significant performance benefits as comparedto conventional biostimulator designs. In particular, the design showedsignificant improvements in tests designed to assess torque-out for thebiostimulator. During the test, biostimulators including primary helicesand having varying suture designs were equally implanted (1.5 turns)within porcine atrial tissue. A counter-rotational torque was thenapplied and measured until disengagement of the biostimulator occurred.Notably, biostimulators including a primary helix and distal andproximal lateral sutures in accordance with the foregoing description ofthe biostimulator 2000 exhibited, on average, a greater than 300%increase in torque-out value as compared to conventional leadlesspacemaker designs. Moreover, designs including both proximal and lateralsutures exhibited approximately 50% better torque-out values as comparedto designs including distal sutures alone.

The specific configuration of the biostimulator 2000 illustrated inFIGS. 20-22 and described above in more detail exhibited strongperformance characteristics during testing. Nevertheless, while thebiostimulator 2000 represents one possible arrangement of components, itis provided merely as an example intended to illustrate one specificconfiguration in accordance with this disclosure. As describedthroughout this disclosure, other arrangements may be possible and mayexhibit other advantages and favorable performance characteristics.Accordingly, the scope of this disclosure should not be limited to thespecific implementation of FIGS. 20-22.

FIGS. 23-25 illustrate another example biostimulator 2300 incorporatingvarious concepts previously described in this disclosure. Morespecifically, FIG. 23 is a first isometric view of the biostimulator2300, FIG. 23 is a second isometric view of the biostimulator 2300focusing on a distal end of the biostimulator 2300. FIG. 25 is a distalview of the biostimulator 2300.

Although other configurations are possible, the biostimulator 2300illustrates one specific configuration found to have advantageousperformance characteristics during testing. Accordingly, to the extentspecific characteristics of the biostimulator 2300 and its componentsare provided below, such discussion is intended to be non-limiting andto provide merely one example of a biostimulator in accordance with thisdisclosure.

Referring first to FIGS. 23-24, the biostimulator 2300 includes ahousing 2302 and a header assembly 2304 coupled thereto. The headerassembly 2304 generally includes a primary fixation element 2305 in theform of a primary helix 2305 that extends about a longitudinal axis 2301of the biostimulator 2300. The primary helix 2305 may be formed ofsimilar materials and have similar dimensional characteristics aspreviously discussed in the context of FIG. 2. As illustrated in FIG.23, the primary helix 2305 is coupled to the header assembly 2304 suchthat the primary helix 2305 extends distally beyond a distal face 2307of the header assembly 2304. The distal face 2307 may correspond to adistal end of the cap 2308 of the header assembly 2304.

The header assembly 2304 further includes distal sutures 2312A, 2312Bthat extend from the distal face 2307 in at least a partially distaldirection and proximal lateral sutures 2313A-2313C that extend laterallyfrom a helix mount 2306 of the header assembly 2304 proximal the distallateral sutures 2312A, 2312B. As described below in more detail, each ofthe distal sutures 2312A, 2312B and the proximal lateral sutures2313A-2313C extend at an angle opposite the direction of the primaryhelix 2305.

The primary helix 2305 is substantially similar to the primary helix2005 described above in the context of the biostimulator 2000.Specifically, the primary helix 2305 extends approximately 1.5 turnsfrom a distal end 2350 of the helix mount 2306 and is formed of a wirehaving a diameter of approximately 0.016 inches and a pitch ofapproximately 0.032 inches. The primary helix 2305 may be formed oftype-302 stainless steel or similar biocompatible material.

The primary helix 2305 includes a sharpened tip 2352 that may be formed,for example, by a double-bevel and that is disposed on an outerperimeter of the wire forming the primary helix 2305. As shown, thesharpened tip 2352 is disposed at approximately a “9 o'clock” position.In other words, the sharpened tip 2352 is disposed on a lateral extentof the primary helix 2305 relative to the longitudinal axis 2301.

As illustrated in each of FIGS. 23-25, the biostimulator 2300 includes apair of distal sutures 2312A, 2312B that extend from a cap 2308 disposedat a distal end of the biostimulator 2300. The distal sutures 2312A,2312B are distributed about the cap 2308 such that they are disposedapproximately 180 degrees apart from each other. The distal sutures2312A, 2012B are also disposed at locations that are offsetapproximately 90 degrees and 270 degrees, respectively, from the tip2352 of the primary helix 2305.

In the specific example of the biostimulator 2300, the distal sutures2312A, 2312B are #3-0 sutures having a diameter ranging betweenapproximately 0.2 mm and 0.3 mm and are formed from polypropylene. Asillustrated in FIG. 25, each of the distal sutures 2312A, 2312B extendsfrom the distal face 2307 in a direction that is at least partially inthe distal direction. The extent to which each of the distal sutures2312A, 2312B extend from the distal face 2307 may vary, however, in theillustrated implementation, each of the distal sutures 2312A, 2312Bincludes a respective tip 2315A, 2315B that extends approximately 0.045inches (+/−0.005 inches) from the cap 2308. In other implementations,the sutures 2312A, 2312B may extend from the distal cap 2008 by adistance from and including 0.020 inches to and including 0.080 inches.

As previously noted, each of the distal sutures 2312A, 2312B extends atleast partially in a distal direction. As illustrated in FIGS. 24A-B andwith reference to the distal suture 2312A, the extent to which thedistal sutures 2312A, 2312B extend in the distal direction may bedefined by an angle ψ relative to a plane 2333 (FIGS. 24A-B) transverseto the longitudinal axis 2301 of the biostimulator 2300. In the specificexample of the biostimulator 2300, the angle ψ is approximately 60degrees. In other implementations, the angle ψ may have any othersuitable value such as from and including 15 degrees to and including 75degrees.

Each of the distal sutures 2312A, 2312B may also be “swept” in adirection counter to that of the primary helix 2305. For example, thesuture 2312A extends at an angle of approximately 30 degrees relative toa first distal radial line 2309A extending from the longitudinal axis2301 through an origin 2311A of the suture 2312A, the origin 2311Acorresponding to a location of the cap 2308 from which the suture 2312Aemerges. Similarly, the suture 2312B also extends at an angle ofapproximately 30 degrees relative to a second distal radial line 2309Bextending from the longitudinal axis 2301 through an origin 2311B of thesuture 2312B, the origin 2311B corresponding to a location of the cap2308 from which the suture 2312B emerges. In other implementations, theproximal sutures 2313A-2313C may extend relative to their respectiveproximal radial lines 2323A-2323C at an angle from and including 15degrees to and including 75 degrees.

In an embodiment, each filament tip can be shaped to have a filamentface along the respective filament axis 1602, which is at an angle tothe filament axis. For example, as shown, each tip 2317A-2317C can becut or trimmed at approximately a 30 degree angle.

As previously discussed, in addition to the distal sutures 2312A, 2312B,the leadless biostimulator 2300 further includes a set of proximallateral sutures 2313A-2313C extending from the helix mount 2306. Theproximal lateral sutures 2313A-2313C are evenly distributed about thehelix mount 2306 such that they are disposed 120 degrees apart from eachother. The proximal lateral sutures 2313A-2313C are also disposed atlocations that are offset approximately 60 degrees, 180 degrees, and 300degrees, respectively, from the tip 2352 of the primary helix 2305. Inanother example implementation, the proximal lateral sutures 2313A-2313Care disposed at locations that are offset approximately 45 degrees, 165degrees, and 285 degrees, respectively, from the tip 2352 of the primaryhelix 2305. In still other implementations, the proximal lateral sutures2313A-2313C may instead be disposed at locations that are offset fromand including 30 degrees to and including 70 degrees, from and including150 degrees to and including 190 degrees, and from and including 270degrees to and including 320 degrees, respectively, from the tip 2352 ofthe primary helix 2305. As illustrated in FIGS. 24A-B, the proximallateral sutures 2313A-2313C may be offset from the origins 2311A, 2311Bof the distal sutures 2312A, 2312B by a distance 2321 of approximately0.01 inches. In other implementations, the distance 2321 may be from andincluding 0.005 inches to and including 0.050 inches. Also, asillustrated in FIGS. 24A-B, the longitudinal distance between the distalsutures 2312A, 2312B and the proximal lateral sutures 2313A-2313C mayvary for each of the proximal lateral sutures 2313A-2313C. For example,the helix mount 2306 may have a helical shape and the proximal lateralsutures 2313A-2313C may extend at various points along the helical shapesuch that each of the proximal lateral sutures 2313A-2313C is disposedat a different longitudinal location.

In the specific example of the biostimulator 2000, the proximal lateralsutures 2313A-2313C are #4-0 sutures having a diameter ranging betweenapproximately 0.15 mm and 0.2 mm and are formed from nylon. Asillustrated in FIG. 25, each of the proximal lateral sutures 2313A-2313Cextends laterally from the helix mount 2306 such that a tip 2317A-2317Cof each suture 2313A-2313C extends approximately 0.020 inches (+/−0.005inches) from the helix mount 2306. In other implementations, the sutures2313A-2313C may extend from the helix mount 2306 by a distance from andincluding 0.010 inches to and including 0.030 inches. The suture 2313Aextends at an angle of approximately 30 degrees relative to a firstproximal radial line 2323A extending from the longitudinal axis 2301through an origin 2317A of the suture 2313A, the origin 2317Acorresponding to a location of the helix mount 2306 from which thesuture 2313A emerges. The suture 2313B also extends at an angle ofapproximately 30 degrees relative to a second proximal radial line 2323Bextending from the longitudinal axis 2301 through an origin 2317B of thesuture 2313B, the origin 2317B corresponding to a location of helixmount 2306 from which the suture 2312B emerges. Finally, the suture2317C extends at an angle of approximately 30 degrees relative to athird proximal radial line 2323C extending from the longitudinal axis2305 through an origin 2317C of the suture 2313C, the origin 2319Ccorresponding to a location of helix mount 2306 from which the suture2313C emerges. In other implementations, the proximal sutures2313A-2313C may extend relative to their respective proximal radiallines 2323A-2323C at an angle from and including 15 degrees to andincluding 75 degrees. As shown, each tip 2317A-2317C is also cut ortrimmed at approximately a 30 degree angle relative to the respectiveradial line 2315A-2315C.

The specific configuration of the biostimulator 2300 illustrated inFIGS. 23-25 and described above in more detail exhibited strongperformance characteristics during testing. Nevertheless, while thebiostimulator 2300 represents one possible arrangement of components, itis provided merely as an example intended to illustrate one specificconfiguration in accordance with this disclosure. As describedthroughout this disclosure, other arrangements may be possible and mayexhibit other advantages and favorable performance characteristics.Accordingly, the scope of this disclosure should not be limited to thespecific implementation of FIGS. 23-25.

As noted above, screwing fixation element 205 into target tissue untiltissue is captured at the pinch point 2110 can increase resistance totorque-out. Furthermore, mounting fixation element 205 on helix mount206 such that the wire extends approximately 1.5 turns from the distalend of the helix mount 206 provides a balance between the ease withwhich the primary helix 205 may be made to engage the wall of the heartand the strength of such engagement. More particularly, it has beendetermined that mounting fixation element 205 on helix mount 206 suchthat wire extends about 1.5 turns from the pinch point 2110, can improvesecurement of biostimulator 100 to the target tissue. As describedbelow, methods of manufacture and device features can be implemented toachieve this benefit.

Referring again to FIG. 21A, helix mount 2006 can include a marking todefine a range within which leading point 2052 can be aligned to set apredefined clocking of fixation element 2005 on biostimulator 2000. Forexample, helix mount flange 2160 can include one or more marks 2150 thatdefine an alignment range 2152. Alignment range 2152 may be acircumferential range, measured along the circumference or outer surfaceof helix mount flange 2160. The circumferential range can be between aleftward boundary and a rightward boundary of the marks 2150. Forexample, mark(s) 2150 may include two marks, e.g., a leftward mark thatis laser marked on helix mount flange 2160 and a rightward mark that islaser marked on helix mount flange 2160 (FIG. 21A). The leftward markwould have a leftward edge defining the leftward boundary and therightward mark would have a rightward edge defining the rightwardboundary. Alternatively, mark(s) 2150 may include a single mark, e.g.,laser marked on helix mount flange 2160, having a leftward edge definingthe leftward boundary and a rightward edge defining the rightwardboundary. (FIG. 24A).

In an embodiment, mark(s) 2150 provide a manufacturing aid to ensurethat leading point 1052 (2052) is the correct number of turns from thedistal end of the helix mount. For example, during manufacturing thehelix mount 2006 having mark(s) 2150 can be mounted on housing 102. Themark(s) define an alignment range 2152. In an operation, the fixationelement 2005 can be screwed onto the helix mount 2006. For example, thecoiled wire can be threaded into the groove of helix mount flange 2160until the leading point 2052 of the wire is aligned with the alignmentrange 2152.

As shown in FIGS. 21B and 24B, in the aligned state, the wire offixation element 205 can extend over 1.4 to 1.6 turns, e.g., 1.455 to1.555 turns of the helical axis 1001 from pinch point 2110 to theleading point. For example, when the leading point is vertically alignedwith alignment range 2152, the wire can extend over 1.4 to 1.6 turns,e.g., between 1.455 to 1.555 turns, e.g., 1.5 turns, of the helical axisfrom the distal end of helix mount flange 2160. Vertical alignment canrefer to the leading point being within an arc volume located between afirst vertical plane radiating from the longitudinal axis and extendingthrough the leftward boundary of mark(s) 2150 and a second verticalplane radiating from the longitudinal axis and extending through therightward boundary of mark(s) 2150. When the leading point is alignedwithin such an arc volume, biostimulator 2300 can be configured suchthat screwing fixation element 2305 into target tissue more than 1.5turns will cause tissue to be sheared and clamped at pinch point 2110.The pinch point 2110 is located 180° away from the tip of the helix2052.

In addition to their use as manufacturing aids, mark(s) 2150 may also beused to promote accurate implantation of biostimulator 100 in a clinicalsetting. In an embodiment, mark(s) 2150 are radiopaque. For example,mark 2150 can be printed or painted on helix mount 206 using aradiopaque ink. Alternatively, mark 2150 can be a radiopaque inlay, suchas a tantalum or platinum post or plate, which can be bonded or pressfit into helix mount 206. In any case, the radiopaque mark 2150 can beimaged under fluoroscopy and/or using another imaging modality to allowan operator to view a position of mark 2150 when biostimulator 100 isimplanted within a target anatomy.

In an embodiment, a method of using biostimulator 100 includesimplanting the biostimulator as shown and described with respect toFIGS. 6A-6B. When the biostimulator is advanced to the target tissue andcontact is made between the tissue and fixation element 105, theoperator can take note of the location of the radiopaque mark 2150. Tosecure biostimulator 100 to the tissue, the operator can effect rotationof the biostimulator such that the fixation element engages and screwsinto the target tissue. During rotation of the biostimulator, theoperator can monitor the number of rotations of mark 2150 underfluoroscopy. When an intended number of rotations of the mark isobserved, the operator can cease turning of biostimulator 100 anddisengage the biostimulator to leave it implanted in the target tissue.

It is noted that the number of turns of biostimulator 100, as controlledby the number of rotations of mark 2150, may be a degree of rotationbetween the leading point and the pinch point of the biostimulator,e.g., 1.5 turns. The operator may, however, turn the biostimulator to amore or lesser degree depending on the particular patient. For example,when the patient has friable tissue, the operator may choose to rotatethe biostimulator by 1.25 turns instead of 1.5 turns. The visibility ofmark 2150 under clinical imaging modalities allows such accurate controlof implantation to occur.

Referring to FIG. 31, an exploded cross-sectional view of a distal endof the biostimulator is shown in accordance with the present disclosure.Helix mount 206 can include cap 208 and helix mount flange 2160. The cap208 can have a distal face 207, which can be a forward facing surface ofcap 208. In an aspect, the distal face 207 can be smooth. For example,the entire surface of distal face 207 can be without an edge, a burr, ora discontinuity. The smoothness of the distal face 207 may result fromthe structure described below.

In an embodiment, mounting flange 2160 includes an energy director 3101that extends distally from a flange base 3103. The flange base 3103 caninclude internal threads to mount to threaded section 704.

In an embodiment, cap 208 includes a boss 3107, which may be aconsumptive or deformable area for creating a weld between cap 208 andenergy director 3101. Cap 208 may have a rounded distal face 207, and athrough-hole extending longitudinally through the face. The through-holecan have an inner diameter 3105 that is slightly larger than electrode211. By contrast, the inner diameter 3105 may be smaller than an innerdimension of energy director 3101. Accordingly, an end view of distalface 207 may reveal electrode 211 and not energy director 3101. Moreparticularly, boss 3107 can be ultrasonically welded to energy director3101 by pressing cap 208 against an upper edge of the energy directorand applying ultrasonic energy to the components until the material ofthe components fuse together. The fused material forms a weld that bondscap 208 to helix mount flange 2160, thus forming a two-component helixmount 206.

In an aspect, the weld that bonds the several mount components is hiddenfrom view by cap 208. More particularly, the weld and flowed material isproximal of cap 208 and does not flow through inner diameter 3105. As aresult, distal face 207 is continuously smooth and no weld edge ispresent on the smooth surface that could rub against the target tissueover time. Thus, a two-part helix mount 206 having a weld or other bondbetween cap 208 and flange 2160 (and visually hidden by distal face 207)may reduce a likelihood of trauma to tissue over time.

Several embodiments are described by way of summary, and not by way oflimitation, in the following paragraphs.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart. The housing includes a distal face anddefines a longitudinal axis. The leadless biostimulator includes aprimary fixation helix attached to the housing and configured to affixthe housing to the wall of the heart by rotating in a screwingdirection. The primary fixation helix extends distally beyond the distalface and defines a helix radius relative to the longitudinal axis. Theleadless biostimulator includes one or more secondary fixation elementsextending from the distal face. Each of the one or more secondaryfixation elements originate from a respective secondary fixation elementorigin disposed at a respective radius relative to the longitudinalaxis. The respective radius is less than the helix radius.

In the embodiment, the one or more secondary fixation elements arecomposed of a flexible biocompatible material.

In the embodiment, the flexible biocompatible material is chosen from agroup consisting of polypropylene, polyethylene, polyester,polyurethane, silicone, poly(lactic acid), poly(glycolic acid),polyimide, polyether ether ketone, and polycarbonate.

In the embodiment, the flexible biocompatible material is a naturalmaterial.

In the embodiment, the flexible biocompatible material is chosen from agroup consisting of, hair, horse hair, nail, hide, horn, sharkskin, andplant fiber.

In the embodiment, the one or more secondary fixation elements includesfrom one to eight secondary fixation elements.

In the embodiment, the one or more secondary fixation elements extendbeyond the helix radius.

In the embodiment, the one or more secondary fixation elements extendbetween adjacent turns of the primary fixation helix.

In the embodiment, the primary fixation helix extends from and includesan eighth of a turn to and including two turns beyond the one or moresecondary fixation elements.

In the embodiment, the one or more secondary fixation elements point ina direction substantially opposite the primary fixation helix such thatrotation of the housing in an unscrewing direction causes the one ormore secondary fixation elements to engage the wall of the heart so asto prevent the primary fixation helix from disengaging the wall of theheart.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart. The housing includes a distal face anddefines a longitudinal axis. The leadless biostimulator includes aprimary fixation helix attached to the housing and configured to affixthe housing to the wall of the heart by rotating in a screwingdirection. The primary fixation helix extends distally beyond the distalface and defines a helix radius relative to the longitudinal axis. Theleadless biostimulator includes one or more secondary fixation elementsextending from the distal face. Each of the one or more facing secondaryfixation elements originates from a respective secondary fixationelement origin disposed at a respective secondary fixation elementradius relative to the longitudinal axis. The respective secondaryfixation element radius is less than the helix radius. The one or moresecondary fixation elements extend along a secondary fixation elementplane perpendicular to the longitudinal axis.

In the embodiment, each of the one or more secondary fixation elementsextends at an angle relative to a line extending between thelongitudinal axis and its respective secondary fixation element origin.The angle is from and including 15 degrees to and including 75 degrees.

In the embodiment, the one or more secondary fixation elements extendbeyond the helix radius.

In the embodiment, the one or more secondary fixation elements extendbetween adjacent turns of the primary fixation helix.

In the embodiment, the one or more secondary fixation elements arecomposed of a flexible biocompatible material.

In the embodiment, the flexible biocompatible material is chosen from agroup consisting of polypropylene, polyethylene, polyester,polyurethane, silicone, poly(lactic acid), poly(glycolic acid),polyimide, polyether ether ketone, and polycarbonate.

In the embodiment, the flexible biocompatible material is chosen from agroup consisting of hair, horse hair, nail, hide, horn, sharkskin, andplant fiber.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart. The housing includes a distal face anddefines a longitudinal axis. The leadless biostimulator includes aprimary fixation helix attached to the housing and configured to affixthe housing to the wall of the heart by rotating in a screwingdirection. The primary fixation helix extends distally beyond the distalface and defines a helix radius relative to the longitudinal axis. Theleadless biostimulator includes one or more secondary fixation elementsextending from the distal face. Each of the one or more secondaryfixation elements originates from a respective secondary fixationelement origin disposed at a respective secondary fixation elementradius relative to the longitudinal axis. The respective secondaryfixation element radius is less than the helix radius. The one or moresecondary fixation elements extend distally, at least in part, from thedistal face.

In the embodiment, each of the one or more secondary fixation elementsextends at an angle relative to a line extending between thelongitudinal axis and its respective secondary fixation element origin.The angle is from and including 15 degrees to and including 75 degrees.

In the embodiment, each of the one or more secondary fixation elementsextends in the distal direction at an angle from and including 15degrees to and including 75 degrees.

In the embodiment, the one or more secondary fixation elements extendbeyond the helix radius.

In the embodiment, the one or more secondary fixation elements extendbetween adjacent turns of the primary fixation helix.

In the embodiment, the one or more secondary fixation elements arecomposed of a flexible biocompatible material chosen from a groupconsisting of polypropylene, polyethylene, polyester, polyurethane,silicone, poly(lactic acid), poly(glycolic acid), polyimide, polyetherether ketone, and polycarbonate.

In the embodiment, the flexible biocompatible material is chosen from agroup consisting of, hair, horse hair, nail, hide, horn, sharkskin, andplant fiber.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart. The leadless biostimulator includes aprimary fixation helix attached to the housing and configured to affixthe housing to the wall of the heart by rotating in a screwingdirection. The leadless biostimulator includes one or more side orforward facing secondary fixation elements extending distally from thehousing. The one or more side or forward facing secondary fixationelements are configured to point in a direction substantially oppositethe primary fixation helix such that rotation of the housing in anunscrewing direction causes the one or more side or forward facingsecondary fixation elements to engage the wall of the heart so as toprevent the primary fixation helix from disengaging the wall of theheart.

In the embodiment, the one or more secondary fixation elements arecomposed of a flexible biocompatible material.

In the embodiment, the flexible biocompatible material is chosen from agroup consisting of polypropylene, polyethylene, polyester,polyurethane, silicone, poly(lactic acid), poly(glycolic acid),polyimide, polyether ether ketone, polycarbonate, hair, horse hair,nail, hide, horn, sharkskin, and plant fiber.

In the embodiment, the one or more secondary fixation elements have adiameter from and including 0.003 inches to and including 0.03 inches.

In the embodiment, the one or more side or forward facing secondaryfixation elements includes from one to eight secondary fixationelements.

In the embodiment, the housing defines a longitudinal axis, and eachsecondary fixation element of the one or more secondary fixationelements further defines: a first axis parallel to the longitudinal axisextending through an origin of the secondary fixation element, a secondaxis perpendicular to the first axis, the second axis extending from theorigin to the longitudinal axis, and a third axis perpendicular to eachof the longitudinal axis and the second axis. The secondary fixationelement extends from the origin at an angle α with respect to the firstaxis and an angle β with respect to the third axis, α being from andincluding 10 degrees to and including 50 degrees and β being from andincluding 15 degrees to and including 75 degrees.

In the embodiment, each secondary fixation element of the one or moresecondary fixation elements extends from and including 0.01 inches toand including 0.3 inches from its respective origin.

In the embodiment, the primary fixation helix extends from and includes0.25 turns to and including 3 turns from the housing and has a wirediameter from and including 0.003 inches to and including 0.03 inches, apitch diameter from and including 0.06 inches to and including 0.3inches, and a pitch from and including 0.01 inches to and including 0.05inches.

In the embodiment, the leadless biostimulator further includes one ormore lateral secondary fixation elements extending laterally relative toa longitudinal axis defined by the housing. The one or more forwardlateral secondary fixation elements configured to point in a directionsubstantially opposite the primary fixation helix such that rotation ofthe housing in the unscrewing direction causes the lateral secondaryfixation elements to engage the wall of the heart so as to prevent theprimary fixation helix from disengaging the wall of the heart.

In the embodiment, each of the one or more lateral secondary fixationelements includes a lateral secondary fixation element origin and arespective normal extends from the longitudinal axis to each of the oneor more lateral secondary fixation elements. Each of the one or morelateral secondary fixation elements extends at an angle from andincluding 15 degrees to and including 75 degrees relative to the normal.

In the embodiment, the one or more lateral secondary fixation elementsare disposed in a body coupled to the housing. Each of the one or morelateral secondary fixation elements extends from and includes 0.003inches to and including 0.05 inches from the body.

In the embodiment, the primary fixation helix defines an inner diameterand at least one of the one or more side or forward facing secondaryfixation elements extends beyond the inner diameter such that duringunscrewing of the primary fixation helix after fixation of the primaryfixation helix to the wall of the heart, the at least one secondaryfixation element interferes with the primary fixation helix.

In the embodiment, each of the one or more secondary fixation elementshas a Young's Modulus from and including 0.5 gigapascals to andincluding 10 gigapascals.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart having a wall, is provided. The leadlessbiostimulator includes a housing sized and configured to be implantedwithin the heart. The leadless biostimulator includes a header assemblycoupled to the housing. The header assembly includes a helix mounthaving a primary fixation helix configured to affix the housing to thewall of the heart by rotating in a screwing direction. The headerassembly includes a flange coupled to the housing and the helix mount,the flange extending through the primary fixation helix. The headerassembly includes a cap including one or more side or forward facingsecondary fixation elements extending laterally or distally from theheader assembly, the one or more side or forward facing secondaryfixation elements configured to point in a direction substantiallyopposite the primary fixation helix such that rotation of the housing inan unscrewing direction causes the side or forward facing secondaryfixation elements to engage the wall of the heart so as to prevent theprimary fixation helix from disengaging the wall of the heart.

In the embodiment, the one or more secondary fixation elements arecomposed of a flexible biocompatible material.

In the embodiment, the housing defines a longitudinal axis, and eachsecondary fixation element of the one or more secondary fixationelements further defines: a first axis parallel to the longitudinal axisextending through an origin of the secondary fixation element; a secondaxis perpendicular to the first axis, the second axis extending from theorigin to the longitudinal axis; and a third axis perpendicular to eachof the longitudinal axis and the second axis. The secondary fixationelement extends from the origin at an angle α with respect to the firstaxis and an angle β with respect to the third axis, α being from andincluding 10 degrees to and including 50 degrees and β being from andincluding 15 degrees to and including 75 degrees.

In the embodiment, the leadless biostimulator further includes one ormore lateral secondary fixation elements coupled to the helix mount. Theone or more lateral secondary fixation elements extend laterally fromthe helix mount and point in a direction substantially opposite theprimary fixation helix such that rotation of the housing in theunscrewing direction causes the lateral secondary fixation elements toengage the wall of the heart so as to prevent the primary fixation helixfrom disengaging the wall of the heart.

In the embodiment, the helix mount includes a first orientation featureand the cap includes a second orientation. The first orientation featuremates with the second orientation feature to maintain the one or moresecondary fixation elements in a predetermined position relative to theprimary fixation helix.

In the embodiment, the one or more secondary fixation elements includesat least a first secondary fixation element and a second secondaryfixation element and the cap defines each of a first secondary fixationelement bore through which the first secondary fixation element extends,a second secondary fixation element bore through which the secondsecondary fixation element extends, and a secondary fixation elementgroove extending between the first secondary fixation element bore andthe second secondary fixation element bore. The first secondary fixationelement and the second secondary fixation element are formed from ashared length of secondary fixation element material extending betweenthe first secondary fixation element bore and the second secondaryfixation element bore through the secondary fixation element grove.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart and defines a longitudinal axis. The leadlessbiostimulator includes a primary fixation helix attached to the housingand configured to affix the housing to the wall of the heart by rotatingin a screwing direction. The leadless biostimulator includes two side orforward facing secondary fixation elements extending distally from thehousing. The one or more side or forward facing secondary fixationelements are configured to point in a direction substantially oppositethe primary fixation helix such that rotation of the housing in anunscrewing direction causes the side or forward facing secondaryfixation elements to engage the wall of the heart so as to prevent theprimary fixation helix from disengaging the wall of the heart. Each ofthe side or forward facing secondary fixation elements is composed ofpolypropylene, has a diameter from and including 0.003 inches to andincluding 0.03 inches, has a Young's Modulus from and including 0.5gigapascals to and including 10 gigapascals, defines a first axisparallel to the longitudinal axis extending through an origin of thesecondary fixation element, defines a second axis perpendicular to thefirst axis, the second axis extending from the origin to thelongitudinal axis, and defines a third axis perpendicular to each of thelongitudinal axis and the second axis. The secondary fixation elementextends from the origin from and including 0.01 inches to and including0.3 inches at an angle α with respect to the first axis and an angle βwith respect to the third axis, α being from and including 10 degrees toand including 50 degrees and β being from and including 15 degrees toand including 75 degrees.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart. The leadless biostimulator includes afixation helix formed from a coiled wire. The fixation helix is attachedto a distal end of the housing and is configured to affix the housing tothe wall of the heart by rotating in a screwing direction. The fixationhelix includes a tip formed by three or more bevels. The tip is disposedabout an outer perimeter of the coiled wire.

In the embodiment, the fixation helix defines a helix axis, the three ormore bevels form an edge extending between the tip and an edge basedisposed on the outer perimeter opposite the tip, and the edge extendsat an angle θ away from a vector extending distally from the edge baseparallel to the helix axis, the angle θ being along a plane defined byboth of the helix axis and the vector and measuring from and including 0degrees to and including 180 degrees.

In the embodiment, the angle θ is from and including 90 degrees to andincluding 180 degrees.

In the embodiment, the edge extends at an angle ψ away from the plane,the angle ψ being from and including 10 degrees to and including 60degrees.

In the embodiment, the coiled wire is one of round wire, flattened wire,hypodermic tubing, and plastic wire.

In the embodiment, the fixation helix has a pitch from and including0.007 inches to and including 0.060 inches.

In the embodiment, the fixation helix has a pitch angle from andincluding 2.5 degrees to and including 20 degrees.

In the embodiment, the leadless biostimulator further includes at leastone contact surface that interacts with the wall of the heart when theleadless biostimulator is implanted within the heart. The at least onecontact surface has at least one surface modification treatment appliedthereto. The surface modification treatment modifies at least one of asurface energy, a surface charge, a surface chemistry, or a surfacemorphology of the portion of the fixation helix relative to a substratematerial to which the surface modification treatment is applied.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart. The leadless biostimulator includes afixation helix formed from a coiled wire. The fixation helix is attachedto a distal end of the housing and configured to affix the housing tothe wall of the heart by rotating in a screwing direction. The fixationhelix includes a helical untapered portion terminating in a tapered tip.The tapered tip terminates within a projected volume extending from thehelical untapered portion. The projected volume has a helical shapeconforming to the helical untapered portion.

In the embodiment, the tapered tip is formed by several bevels.

In the embodiment, the several bevels consists of two bevels and thetapered tip terminates along an outer extent of the projected volume.

In the embodiment, the tapered tip terminates from and including 0.002inches to and including 0.02 inches from the untapered portion.

In the embodiment, the tapered tip terminates from and including 0.001inches to and including 0.009 inches from a longitudinal axis defined bythe coiled wire.

In the embodiment, the fixation helix has a pitch from and including0.007 inches to and including 0.060 inches.

In the embodiment, the fixation helix has a pitch angle from andincluding 2.5 degrees to and including 20 degrees.

In the embodiment, the several bevels includes at least three bevels,and the tapered tip terminates: (i) from and including 0.002 inches toand including 0.02 inches from the untapered portion, (ii) from andincluding 0.001 inches to and including 0.009 inches from a longitudinalaxis defined by the coiled wire, and (iii) from and including 90 degreesto and including 160 degrees relative to a tip axis, the tip axisextending perpendicular to the longitudinal axis and parallel to a helixaxis of the fixation helix.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart. The leadless biostimulator includes afixation helix formed from a coiled wire, coupled to a distal end of thehousing, and extending along a helix axis. The fixation helix isconfigured to affix the housing to the wall of the heart by rotating ina screwing direction. The fixation helix includes a tip disposed on anouter perimeter of the coiled wire and formed by three or more bevels.The three or more bevels form an edge extending between the tip and anedge base disposed on the outer perimeter opposite the tip. The edgeextends at an angle θ away from a vector distally extending from theedge base parallel to the helix axis. The angle θ being along a planedefined by the helix axis and the vector and being from and including 90degrees to and including 180 degrees.

In the embodiment, the angle θ is 90 degrees.

In the embodiment, the angle θ is 180 degrees.

In the embodiment, the edge extends at an angle ψ away from the plane,the angle ψ being from and including 10 degrees to and including 60degrees, and the tip has a length from and including 0.002 inches to andincluding 0.02 inches.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart and defining a longitudinal axis. Theleadless biostimulator includes a header assembly coupled to a distalend of the housing and having a distal face. The header assemblyincludes a helix mount. The header assembly includes a fixation helixcoupled to the helix mount and extending distally from the helix mount.The fixation helix is configured to affix the housing to the wall of theheart by rotating in a screwing direction. The header assembly includesseveral distal secondary fixation elements extending laterally from thedistal face and at least partially in a direction opposite the screwingdirection. The header assembly includes several proximal secondaryfixation elements proximal the distal secondary fixation elements. Theproximal secondary fixation elements extend laterally from the helixmount and at least partially in a direction opposite the screwingdirection.

In the embodiment, the fixation helix includes a sharpened tip disposedon a lateral extent of the primary helix relative to the longitudinalaxis.

In the embodiment, the fixation helix extends approximately 1.5 turnsbeyond the distal face of the header assembly.

In the embodiment, the several distal secondary fixation elementsconsists of a first distal secondary fixation element and a seconddistal secondary fixation element. The first distal secondary fixationelement is disposed at a first offset of approximately 90 degreesrelative to a tip of the fixation helix and the second distal secondaryfixation element is disposed at a second offset of approximately 270degrees relative to the tip of the fixation helix.

In the embodiment, each distal secondary fixation element of the severaldistal secondary fixation elements extends at an angle from andincluding 15 degrees to and including 75 degrees relative to arespective radial line extending from the longitudinal axis to arespective origin of the distal secondary fixation element.

In the embodiment, each distal secondary fixation element extends at anangle of approximately 30 degrees relative to the respective radialline.

In the embodiment, each distal secondary fixation element of the severaldistal secondary fixation elements extends from and including 0.020inches to and including 0.080 inches from the distal face.

In the embodiment, each distal secondary fixation element of the severaldistal secondary fixation elements extends approximately 0.045 inchesfrom the distal face.

In the embodiment, each distal secondary fixation element is a #3-0suture.

In the embodiment, the several proximal secondary fixation elementsconsists of a first proximal secondary fixation element, a secondproximal secondary fixation element, and a third proximal secondaryfixation element. The first proximal secondary fixation element isdisposed at a first offset from and including 30 degrees to andincluding 70 degrees relative to a tip of the fixation helix. The secondproximal secondary fixation element is disposed at a second offset offrom and including 150 degrees to and including 190 degrees relative tothe tip of the fixation helix. The third proximal secondary fixationelement is disposed at a third offset of from and including 270 degreesto and including 320 degrees relative to the tip of the fixation helix.

In the embodiment, each proximal secondary fixation element of theseveral proximal secondary fixation elements extends at an angle fromand including 15 degrees to and including 75 degrees relative to arespective radial line extending from the longitudinal axis to arespective origin of the proximal secondary fixation element.

In the embodiment, each proximal secondary fixation element extends atan angle of approximately 30 degrees relative to the respective radialline.

In the embodiment, each proximal secondary fixation element of theseveral proximal secondary fixation elements extends from and including0.010 inches to and including 0.030 inches from the helix mount.

In the embodiment, each proximal secondary fixation element extendsapproximately 0.020 inches from the distal face.

In the embodiment, each proximal secondary fixation element is a #4-0suture.

In the embodiment, the several distal secondary fixation elements islongitudinally offset from the several proximal secondary fixationelements by a distance from and including 0.005 inches to and including0.050 inches.

In the embodiment, the distance is approximately 0.01 inches.

In an embodiment, a leadless biostimulator for implantation within aheart of a patient, the heart including a wall, is provided. Theleadless biostimulator includes a housing sized and configured to beimplanted within the heart, and defining a longitudinal axis. Theleadless biostimulator includes a distal fixation assembly coupled to adistal end of the housing. The distal fixation assembly includes a helixmount including a distal face. The distal fixation assembly includes afixation helix attached to the helix mount and extending distally fromthe helix mount. The fixation helix is configured to affix the housingto the wall of the heart by rotating in a screwing direction. The distalfixation assembly includes several distal secondary fixation elementsextending from the distal face. The distal fixation assembly includesseveral proximal secondary fixation elements proximal the distalsecondary fixation elements and extending laterally from the helix mountin a direction opposite the screwing direction. Each distal secondaryfixation element of the several distal secondary fixation elementsextends in a distal direction at a first angle relative to a planetransverse to the longitudinal axis and opposite the screwing directionat a second angle relative to a respective radial line extending fromthe longitudinal axis to a respective origin of the distal secondaryfixation element. Each of the first angle and the second angle beingfrom and including 15 degrees to and including 75 degrees.

In the embodiment, each of the distal secondary fixation elements is a#3-0 suture and extends from and including 0.020 to and including 0.080inches from the distal face.

In the embodiment, each proximal secondary fixation element of theseveral proximal secondary fixation elements is a #4-0 suture extendingfrom and including 0.010 inches to and including 0.030 inches from thehelix mount at an angle from and including 15 degrees to and including75 degrees relative to a respective radial line extending from thelongitudinal axis to a respective origin of the proximal secondaryfixation element.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. A biostimulator, comprising: a housing having alongitudinal axis and containing an electronics compartment; and afixation element mounted on the housing, wherein the fixation elementincludes a helix extending along a helical axis about the longitudinalaxis to a distal edge, wherein the distal edge extends around thehelical axis and defines one or more helix faces on the helical axis,wherein a transverse plane orthogonal to the longitudinal axisintersects a center of the one or more helix faces, and wherein thehelix includes a leading point on a proximalmost portion of the helixface proximal to the transverse plane.
 2. The biostimulator of claim 1,wherein the helix includes an ellipsoidal outer surface extending aroundthe helical axis, wherein the distal edge is at an intersection betweenthe ellipsoidal outer surface and the one or more helix faces, andwherein the leading point is on the distal edge.
 3. The biostimulator ofclaim 1, wherein a longitudinal plane intersects and is orthogonal tothe transverse plane at the center of the one or more helix faces, andwherein the leading point is at a six o'clock position on thelongitudinal plane.
 4. The biostimulator of claim 3, wherein the sixo'clock position is on the distal edge.
 5. The biostimulator of claim 1,wherein the one or more helix faces includes a plurality of bevel facesconverging at the leading point, wherein the plurality of bevel facesintersect along a leading edge extending from the leading point alongthe one or more helix faces to a base on the distal edge, and whereinthe base is on an opposite side of the transverse plane from thehousing.
 6. The biostimulator of claim 1 further comprising a helixmount having a helix mount flange, wherein the helix mount flangeincludes one or more marks defining an alignment range, wherein thehelix extends distally from a distal end of the helix mount flange tothe leading point, and wherein the leading point is aligned with thealignment range.
 7. The biostimulator of claim 6, wherein the one ormore marks are radiopaque such that the one or more marks are visibleunder fluoroscopy.
 8. The biostimulator of claim 6, wherein thealignment range is between a leftward boundary and a rightward boundaryof the one or more marks, and wherein the helix extends to the leadingpoint that is vertically aligned with the alignment range.
 9. Thebiostimulator of claim 8 further comprising a plurality of backstopelements to resist backward movement of the housing when the fixationelement is engaged in tissue.
 10. The biostimulator of claim 9, whereinthe plurality of backstop elements include a non-metallic filamentextending through a bore in a sidewall of the helix mount along afilament axis to a filament tip.
 11. The biostimulator of claim 10,wherein the filament tip has a filament face on the filament axis, andwherein the filament face is at an angle to the filament axis.
 12. Thebiostimulator of claim 10, wherein the plurality of backstop elementsinclude a plurality of non-metallic filaments extending throughrespective bores in the sidewall, and wherein each bore is at adifferent longitudinal position relative to the longitudinal axis. 13.The biostimulator of claim 10, wherein the non-metallic filamentincludes a natural fiber.
 14. The biostimulator of claim 9, wherein theplurality of backstop elements include a pinch point between the helixand the distal end of the helix mount flange.
 15. The biostimulator ofclaim 14, wherein the biostimulator has a first removal torque when thefixation element is engaged in tissue and the tissue is not at the pinchpoint, wherein the biostimulator has a second removal torque when thefixation element is engaged in the tissue and the tissue is at the pinchpoint, and wherein the second removal torque is at least 10% higher thanthe first removal torque.
 16. The biostimulator of claim 1, wherein thebiostimulator is a leadless cardiac pacemaker.
 17. The biostimulator ofclaim 1 further comprising an active helical electrode mounted on thehousing, wherein the active helical electrode includes an electrodehelix extending along an electrode helical axis about the longitudinalaxis radially inward of the helical axis, and wherein the electrodehelical axis and the helical axis revolve about the longitudinal axis ina same rotational direction.
 18. A method, comprising: mounting a helixmount on a housing, wherein the helix mount includes a helix mountflange, wherein the helix mount flange includes one or more marks on anouter surface of the helix mount flange defining an alignment rangealong the outer surface of the helix mount flange; and screwing afixation element onto the helix mount, wherein the fixation elementincludes a helix extending along a helical axis, and wherein the helixis screwed onto the helix mount flange until a leading point of thehelix is aligned with the alignment range.
 19. The method of claim 18,wherein the one or more marks are radiopaque such that the one or moremarks are visible under fluoroscopy.
 20. The method of claim 19, whereinthe housing includes a longitudinal axis, wherein the alignment rangeincludes an arc volume between a first plane extending from thelongitudinal axis through a leftward boundary of the one or more marksand a second plane extending from the longitudinal axis through arightward boundary of the one or more marks, and wherein the helixextends over about 1.4 to about 1.6 turns of the helical axis to theleading point when the leading point is vertically aligned with thealignment range.
 21. The method of claim 20 further comprising insertinga non-metallic filament through a bore in a sidewall of the helix mount.22. The method of claim 21, wherein the helix mount includes a capmounted distally on the helix mount, and wherein the bore is in the cap.23. The method of claim 22, wherein the helix extends along the helicalaxis about the longitudinal axis of the housing to a distal edge,wherein the distal edge extends around the helical axis and defines oneor more helix faces on the helical axis, wherein a transverse planeorthogonal to the longitudinal axis intersects a center of the one ormore helix faces, and wherein the leading point is proximal to thetransverse plane.
 24. The method of claim 23, wherein the helix includesan ellipsoidal outer surface extending around the helical axis, whereinthe distal edge is at an intersection between the ellipsoidal outersurface and the helix face, and wherein the leading point is on thedistal edge.
 25. A method, comprising: advancing a biostimulator to atarget tissue, wherein the biostimulator includes a housing having alongitudinal axis and containing an electronics compartment, and afixation element mounted on a helix mount flange of the housing, whereinthe fixation element includes a helix extending from a distal end of thehelix mount flange to a leading point, wherein the helix mount flangeincludes one or more radiopaque marks on an outer surface of the helixmount flange defining an alignment range along the outer surface of thehelix mount flange, and wherein the leading point is aligned with thealignment range; viewing the one or more radiopaque marks underfluoroscopy; and rotating the biostimulator until the fluoroscopicallyviewed one or more radiopaque marks complete a predetermined number ofturns.
 26. The method of claim 25, wherein the predetermined number ofturns is about 1.5 turns.